Latin 2 Quiz: 3rd Declension Nouns
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3rd Declension NounsQuestion 1 of 10

A student is asked to identify the form urbium and explain why it cannot belong to the same paradigmatic slot as regum. Which of the following analyses is most accurate?

Urbium is the genitive plural of urbs, urbis (an i-stem 3rd-declension noun) and takes the ending -ium rather than -um; regum is the genitive plural of rex, regis (a non-i-stem 3rd-declension noun) and takes -um, so the two forms occupy the same case and number but differ because of i-stem vs. non-i-stem classification.
Urbium is the dative plural of urbs, urbis and takes -ium as a dative marker; regum is the genitive plural of rex, regis and takes -um, so the two forms differ in both case and the subclass of 3rd declension to which they belong.
Urbium is the genitive plural of urbs, urbis and takes -ium because all 3rd-declension nouns whose nominative ends in a consonant cluster use -ium in every plural case; regum uses -um because rex ends in a single consonant in the nominative, making it a regular non-i-stem with no plural variations.
Urbium is the ablative plural of urbs, urbis and takes -ium as a variant ablative ending found only in i-stem nouns; regum is the genitive plural of rex, regis with the standard -um ending, so the forms differ in case, number, and declension subtype.
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Latin 2 Quiz

Latin 2 Quiz: 3rd Declension Nouns

Practice 3rd Declension Nouns in Latin 2 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on 3rd Declension Nouns, giving you a quick way to practice the rules, question types, and explanations that matter most for Latin 2.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A student is asked to identify the form urbium and explain why it cannot belong to the same paradigmatic slot as regum. Which of the following analyses is most accurate?

  1. Urbium is the genitive plural of urbs, urbis (an i-stem 3rd-declension noun) and takes the ending -ium rather than -um; regum is the genitive plural of rex, regis (a non-i-stem 3rd-declension noun) and takes -um, so the two forms occupy the same case and number but differ because of i-stem vs. non-i-stem classification. (correct answer)
  2. Urbium is the dative plural of urbs, urbis and takes -ium as a dative marker; regum is the genitive plural of rex, regis and takes -um, so the two forms differ in both case and the subclass of 3rd declension to which they belong.
  3. Urbium is the genitive plural of urbs, urbis and takes -ium because all 3rd-declension nouns whose nominative ends in a consonant cluster use -ium in every plural case; regum uses -um because rex ends in a single consonant in the nominative, making it a regular non-i-stem with no plural variations.
  4. Urbium is the ablative plural of urbs, urbis and takes -ium as a variant ablative ending found only in i-stem nouns; regum is the genitive plural of rex, regis with the standard -um ending, so the forms differ in case, number, and declension subtype.
Explanation: When you encounter a question comparing Latin noun endings, your first instinct should be to identify each word's case, number, and declension subtype before drawing any conclusions about why their endings differ. Urbium is the genitive plural of urbs, urbis (3rd declension, i-stem), and regum is the genitive plural of rex, regis (3rd declension, non-i-stem). Both forms occupy the same paradigmatic slot — genitive plural — but arrive there with different endings precisely because of i-stem classification. I-stem nouns like urbs take -ium in the genitive plural, while non-i-stem nouns like rex take -um. This makes A the accurate analysis: same case and number, different endings due to subclass membership. B is wrong because it misidentifies urbium as dative plural — the dative plural of urbs is urbibus, not urbium. Calling them different cases compounds this error. C contains a kernel of truth (consonant clusters do signal i-stems) but then wildly overstates the rule by claiming i-stems use -ium "in every plural case," which is false. Only the genitive plural consistently shows -ium; other cases vary. D misidentifies urbium as ablative plural. The ablative plural of urbs is urbibus, so this is simply wrong on case identification. Study tip: Memorize the three i-stem signals — monosyllabic nominatives with a consonant cluster (like urbs), neuters ending in -e/-al/-ar, and nouns with equal syllable counts in nominative and genitive. Whenever you see -ium in a 3rd-declension noun, that's your i-stem flag.

Question 2

Consider the following list of Latin forms: patris, patrī, patrem, patre, patres, patrum, patribus.

A student claims that patrī and patrēs are both ambiguous forms — that is, each could represent more than one case. Which of the following evaluations of the student's claim is correct?

  1. The student is half correct: patrēs is genuinely ambiguous (serving as both nominative and accusative plural in 3rd-declension consonant-stem nouns), but patrī is unambiguous (dative singular only); the student is right about patrēs but wrong about patrī. (correct answer)
  2. The student is correct about patrī (dative or locative singular) but incorrect about patrēs, which serves only as nominative plural and cannot also be accusative plural in a 3rd-declension consonant-stem noun.
  3. The student is correct about both forms: patrī is ambiguous between dative singular and genitive singular, and patrēs is ambiguous between nominative plural and vocative plural, making both forms genuinely two-way ambiguous.
  4. The student is incorrect about both forms: patrī is unambiguous as dative singular only, and patrēs is likewise unambiguous as nominative plural only, because 3rd-declension nouns do not share endings across cases the way 1st- and 2nd-declension nouns do.
Explanation: When analyzing Latin noun forms for ambiguity, your job is to ask: "How many different cases could this ending legitimately represent in this declension?" For pater (3rd-declension consonant-stem), you need to know the full paradigm cold. Starting with patrī: in 3rd-declension nouns, the dative singular ending is -ī, and so is the genitive singular... wait — no. That's the trap. The genitive singular of a consonant-stem noun like pater is patris (not patrī). The -ī genitive belongs to 2nd-declension nouns. So patrī is dative singular only — unambiguous. The student is wrong about this form. Now patrēs: in 3rd-declension nouns, the nominative plural and accusative plural are both formed with -ēs. So patrēs genuinely represents two cases — nominative AND accusative plural. The student is right about this form. That reasoning confirms A: the student is half correct. Patrēs is genuinely ambiguous (nominative/accusative plural), while patrī is unambiguous (dative singular only). Choice B gets it backwards — it accepts the wrong claim (patrī as ambiguous) and rejects the correct one (patrēs as accusative plural). Choice C is doubly wrong: it misidentifies patrī as genitive/dative (a 2nd-declension confusion) and misidentifies patrēs's ambiguity as nominative/vocative rather than nominative/accusative. Choice D incorrectly claims 3rd-declension nouns never share endings — they absolutely do in the plural. Study tip: Keep your declension endings strictly separated. The dative -ī trap (confusing 2nd-declension genitive with 3rd-declension dative) is one of the most common errors on Latin case identification questions.

Question 3

A Latin teacher presents the following paradigm slot to students and asks them to fill it in: the ablative singular of lux, lucis (f., light).

Four students give different answers. Which student gives the correct form AND the correct reasoning?

  1. Student A writes luce and explains that the ablative singular of 3rd-declension consonant-stem nouns is formed by adding -e to the stem luc-, producing luc-e, with no i-stem complications since lux is not parisyllabic. (correct answer)
  2. Student B writes lucis and explains that the ablative singular ending for all 3rd-declension nouns is -is, which is added directly to the stem, making lucis the correct ablative singular form.
  3. Student C writes luci and explains that because lux ends in a consonant cluster (-x = cs), it qualifies as an i-stem noun and therefore takes -ī in the ablative singular instead of the standard -e.
  4. Student D writes lucem and explains that the ablative singular of 3rd-declension feminine nouns always uses the same form as the accusative singular, so lucem serves double duty as both accusative and ablative for nouns like lux.
Explanation: When declining 3rd-declension nouns, your first task is always to identify the stem (by dropping -is from the genitive singular) and then determine whether the noun is a consonant-stem or an i-stem, since this affects certain endings. For lux, lucis, the stem is luc-, and the ablative singular of a standard consonant-stem noun is formed simply by adding -e to that stem. Student A is correct on both counts. Lux is not parisyllabic (the nominative has one syllable, the genitive two), and it does not meet the other i-stem criteria in a way that forces -ī in the ablative. The stem luc- + -e gives you luce, the standard ablative singular for a 3rd-declension consonant-stem noun. Student B confuses the genitive singular ending (-is) with the ablative singular ending. These are completely different cases — -is is never the ablative singular ending in the 3rd declension. Student C applies a real-but-misapplied rule. While -x does represent a consonant cluster (cs), that feature alone does not automatically make a noun an i-stem. True i-stems that take -ī in the ablative are typically parisyllabic nouns or a small set of monosyllabic neuters. Lux doesn't qualify, so luci is wrong here. Student D invents a false rule entirely. The accusative (lucem) and ablative (luce) of 3rd-declension nouns are never identical — don't confuse them. Study tip: Memorize the i-stem checklist (parisyllabic? neuter ending in -al/-ar/-e?) so you always know when to use -ī versus -e in the ablative singular.

Question 4

Examine the following sentence: Voces civium in foro auditae sunt.

A student must parse both voces and civium and explain how each noun's ending reflects its grammatical function. Which of the following analyses is fully correct?

  1. Voces is nominative plural of vox, vocis (3rd-declension consonant-stem), serving as the subject of the passive verb auditae sunt; civium is ablative plural of civis, civis (3rd-declension i-stem), ending in -ium as a variant ablative plural form, and indicates the source from which the voices came.
  2. Voces is accusative plural of vox, vocis (3rd-declension consonant-stem), serving as the direct object of auditae sunt; civium is genitive plural of civis, civis (3rd-declension i-stem), ending in -ium, and modifies voces as a partitive genitive indicating a portion of the citizen body.
  3. Voces is nominative plural of vox, vocis (3rd-declension consonant-stem noun), serving as the subject of the passive verb auditae sunt; civium is genitive plural of civis, civis (3rd-declension i-stem noun), ending in -ium because i-stem nouns take -ium in the genitive plural, and it modifies voces as a possessive genitive ('the voices of the citizens'). (correct answer)
  4. Voces is vocative plural of vox, vocis (3rd-declension i-stem), functioning as a direct address embedded in the sentence; civium is genitive plural of civis, civis, ending in -ium because all 3rd-declension nouns — both i-stem and non-i-stem — use -ium in the genitive plural.
Explanation: When parsing nouns in Latin, you need to identify two things simultaneously: the correct form (declension, case, number) and the grammatical function that form performs in the sentence. This question tests both skills for a 3rd-declension consonant-stem noun and a 3rd-declension i-stem noun. Starting with voces: vox, vocis is a 3rd-declension consonant-stem noun. Its nominative and accusative plural both end in -es, so you must use context to distinguish them. Since auditae sunt is a passive verb ("were heard"), and voces is feminine plural matching the feminine plural participle auditae, voces functions as the subject of that passive verb — nominative plural. This confirms C's analysis. For civium: civis, civis is a 3rd-declension i-stem noun. I-stem nouns are specifically characterized by taking -ium (not -um) in the genitive plural. Here, civium modifies voces as a possessive genitive — "the voices of the citizens." Answer C captures this perfectly. Answer A fails on civium: -ium is the genitive plural ending for i-stems, not a "variant ablative plural form." Ablative plural for i-stems ends in -ibus. Answer B misidentifies voces as accusative — passive verbs take a subject, not a direct object, and the participle's gender and number agree with a nominative subject. It also misreads civium as partitive rather than possessive genitive. Answer D makes two errors: voces is not vocative (there's no address occurring in this sentence), and it falsely claims all 3rd-declension nouns use -ium in the genitive plural — only i-stems do. Your study tip: always pair case identification with function. When you see -ium in a 3rd-declension noun, ask yourself whether it's an i-stem — that ending is a hallmark diagnostic feature you should memorize.

Question 5

A student encounters the noun tempus, temporis (n., time) and needs to produce its dative singular form. The student knows that the stem is tempor-. Which of the following forms is correct, and why might a student incorrectly produce one of the other forms?

  1. Tempore is the correct dative singular; neuter 3rd-declension nouns use -e for the dative singular (the same ending as the ablative), and a student might mistakenly write tempori by applying the masculine dative rule to a neuter noun.
  2. Temporis is the correct dative singular; the dative singular of neuter 3rd-declension nouns is identical to the genitive singular, so temporis serves both functions, and a student might mistakenly write tempori by overapplying the masculine/feminine dative rule.
  3. Tempori is the correct dative singular; the dative singular of 3rd-declension nouns is formed by adding -ī to the stem (tempor- + -ī), and a student might incorrectly write temporis by confusing the dative -ī ending with the genitive -is ending. (correct answer)
  4. Temporī is the correct dative singular, but only because tempus is a neuter i-stem noun whose stem ends in -or-; standard non-i-stem neuters would instead take -e in the dative singular, making tempore the expected form for regular neuter nouns.
Explanation: When declining a 3rd-declension noun like tempus, temporis, your first step is always to isolate the stem from the genitive singular — here, temporis → stem tempor-. Then apply the correct case ending. For the dative singular, all 3rd-declension nouns (masculine, feminine, and neuter) add -ī to the stem, giving you tempor- + -ī = temporī. This makes C the correct answer. Understanding why the wrong answers fail is just as important here. A invents a rule that doesn't exist — neuter 3rd-declension nouns do not use -e for the dative singular. The ending -e appears in the ablative singular of most 3rd-declension nouns (tempore = "by/from time"), not the dative. Confusing dative and ablative endings is one of the most common errors in Latin. B claims the dative is identical to the genitive singular (temporis), which is simply false; temporis means "of time" (genitive), not "to/for time" (dative). These two cases have distinct endings and must not be blurred together. D fabricates a category — "tempus as a neuter i-stem" — that does not apply. Tempus is a standard consonant-stem neuter, and the dative singular -ī ending applies to all 3rd-declension nouns equally, regardless of gender or i-stem status. Your best study strategy: memorize the full 3rd-declension singular endings as a unit. Notice that -ī (dative) and -is (genitive) look similar but are distinct — drilling paradigms like tempus, temporis, temporī, temporem, tempore will prevent you from mixing them up under pressure.

Question 6

A student is given the following list and told that exactly two of the five forms are nominative plural: leges, legum, legibus, lege, legēs.

The student must identify which two forms are nominative plural and explain how they differ from the other forms in the list. Which of the following responses is correct?

  1. The two nominative plurals are leges and legibus: leges is the standard nominative plural ending for 3rd-declension nouns, while legibus serves as an alternative nominative plural in formal or archaic registers of Latin prose.
  2. Only one form is nominative plural: legēs (leges); legum is genitive plural, legibus is dative/ablative plural, lege is ablative singular, and the list contains only one nominative plural. The original claim that two forms are nominative plural is therefore false for this paradigm. (correct answer)
  3. The two nominative plurals are leges and lege: leges is the standard nominative plural of lex, and lege is an alternative nominative plural form used when the noun follows a preposition in the nominative clause.
  4. The two nominative plurals are leges and legum: leges is the nominative plural, and legum also functions as nominative plural in archaic Latin, where the genitive plural ending -um was originally a shared marker for nominative and genitive in consonant-stem nouns.
Explanation: When you see a question about Latin noun forms, your first move should be to run through the full paradigm mentally. For lex, legis (a 3rd-declension consonant-stem noun), the complete set of forms looks like this: nominative singular lex, genitive singular legis, dative singular legi, accusative singular legem, ablative singular lege, nominative plural legēs, genitive plural legum, dative/ablative plural legibus, accusative plural legēs. Notice something immediately: the nominative plural and accusative plural are identical — both are legēs. This means the list contains only one distinct form that is nominative plural, not two. Answer B is correct precisely because it calls out the false premise. Legēs (written leges in the list) is the nominative plural, but it is also the accusative plural — the list does not contain two different nominative plural forms. Every other form belongs to a different category: legum is genitive plural, legibus is dative/ablative plural, and lege is ablative singular. Answer A is wrong because legibus is never a nominative plural form — it is exclusively dative/ablative plural. There is no archaic or formal register where this changes. Answer C is wrong because lege is the ablative singular, not any kind of nominative plural; no such prepositional nominative variant exists in Latin. Answer D is wrong because while the -um ending is indeed genitive plural, it never doubles as nominative plural — that claim is a fabrication with no grammatical basis. Your study tip: always write out the full paradigm when a question isolates individual forms. Endings look ambiguous in isolation but become clear in context.

Question 7

Consider the following pair of sentences: (1) Rex populum amat. (2) Populus regem amat.

A student claims that in sentence (1), rex is nominative singular and populum is accusative singular, and in sentence (2), populus is nominative singular and regem is accusative singular — and that in both sentences, the subject and object have simply switched. The student then concludes that rex and regem must belong to the same declension as populus and populum. Which of the following best evaluates this conclusion?

  1. The student's parsing is correct, and the conclusion is also correct: both rex/regem and populus/populum belong to the 3rd declension, because all Latin nouns whose accusative singular ends in -em or -um are classified together in the same broad declension category.
  2. The student's parsing of both sentences is correct, but the conclusion is wrong: rex and regem are 3rd-declension forms (stem reg-; nominative rex, accusative singular regem), while populus and populum are 2nd-declension forms (genitive singular populi); sharing a syntactic role does not mean sharing a declension. (correct answer)
  3. The student's parsing of sentence (1) is wrong: rex is actually vocative, not nominative, because 3rd-declension nouns whose nominative ends in -x always shift to a vocative function when placed sentence-initially; the conclusion about declension is therefore also flawed.
  4. The student's parsing is correct for both sentences, and the conclusion is partially correct: rex and regem are 3rd-declension, and populus and populum are also 3rd-declension, because the 2nd declension is reserved exclusively for neuter nouns ending in -um in the nominative singular, not masculine nouns like populus.
Explanation: When you encounter a question about Latin noun forms, always ask two separate questions: (1) What is the syntactic role of this noun? and (2) Which declension does it belong to? These are independent facts, and confusing them is exactly the trap this question sets. The student's parsing is actually sound. In sentence (1), rex is nominative singular (subject: "the king loves") and populum is accusative singular (object: "the people"). In sentence (2), populus is nominative singular and regem is accusative singular — subject and object have indeed switched. So far, so good. The error comes in the conclusion. Just because two nouns share a syntactic role — or even the same case ending — does not mean they belong to the same declension. Rex and regem are 3rd-declension forms: the stem is reg-, the nominative singular adds no vowel ending (giving rex via consonant shift), and the accusative singular takes -em. Meanwhile, populus and populum are 2nd-declension forms, confirmed by the genitive singular populi — the telltale sign of the 2nd declension. B correctly identifies both of these facts and names the real flaw in the student's reasoning. Answer A is wrong because accusative endings -em and -um do not belong to one shared declension — -em is 3rd-declension and -um is 2nd-declension (masculine/neuter). Answer C invents a rule about sentence-initial vocatives that simply does not exist in Latin grammar. Answer D incorrectly claims the 2nd declension is reserved for neuters; masculine nouns like populus, dominus, and amicus are classic 2nd-declension nouns. Your study tip: always check the genitive singular to confirm a noun's declension — it's the most reliable diagnostic, not the nominative or accusative alone.

Question 8

A Roman student encounters the following sentence in a text: Miles fortis hostem vincit. The student wants to determine the grammatical role of hostem in this sentence.

Which of the following correctly identifies the case, number, and declension of hostem, and what is its function in the sentence?

  1. Accusative singular, 3rd declension; it is the direct object of vincit, since the -em ending marks the accusative singular for 3rd-declension nouns. (correct answer)
  2. Ablative singular, 3rd declension; it is the object of an implied preposition, since -em can signal separation or accompaniment in context-driven constructions.
  3. Nominative singular, 3rd declension; it is a predicate nominative linked to miles through vincit, since both nouns refer to combatants in the same action.
  4. Genitive singular, 3rd declension; it modifies miles as a possessive, since 3rd-declension genitives can resemble accusative forms in certain i-stem paradigms.
Explanation: When analyzing an unknown Latin noun form, your first move should always be to identify the ending and match it to a declension paradigm — then confirm the grammatical function that ending signals in context. In Miles fortis hostem vincit, the word hostem comes from hostis, hostis (m.), meaning "enemy," a 3rd-declension noun. The ending -em is the standard accusative singular marker for 3rd-declension nouns (both consonant-stem and i-stem). Since vincit ("conquers/defeats") is a transitive verb requiring a direct object, hostem fills that role perfectly. Answer A is correct: accusative singular, 3rd declension, functioning as the direct object of vincit. Answer B is a tempting trap if you conflate the ablative's range of uses with its actual endings. The ablative singular for 3rd-declension nouns ends in -e (e.g., hoste), not -em. The -em ending never signals ablative, regardless of context. Answer C confuses accusative with nominative. A predicate nominative would require a linking verb (esse, "to be"), not an action verb like vincit. Additionally, the nominative singular of hostis is hostis, not hostem. Answer D is simply a fabrication about genitive forms. The genitive singular of 3rd-declension nouns ends in -is (hostis). No standard Latin paradigm produces a genitive in -em. Study tip: Memorize the 3rd-declension singular endings as a set — nominative (-s or variable), genitive (-is), accusative (-em), ablative (-e) — so you can eliminate wrong cases instantly by their endings alone.

Question 9

Read the following sentence: Dux militibus signum dedit.

A student parses militibus as dative plural and argues that it indicates the recipients of signum. A second student argues militibus must be ablative plural indicating accompaniment. Which of the following statements best resolves this dispute, and what morphological evidence supports the resolution?

  1. Neither student is correct, because militibus is actually a genitive plural form; the genitive plural of 3rd-declension non-i-stem nouns ends in -ibus when the stem ends in a liquid consonant such as the -l- in milit-.
  2. The first student is definitively correct and the second is wrong, because -ibus is exclusively a dative plural ending in all 3rd-declension nouns; the ablative plural of miles would instead end in -ibus only in i-stem nouns, and miles is a non-i-stem.
  3. The second student is correct, because dare never governs a dative indirect object in classical Latin prose; militibus must therefore be ablative of accompaniment, and the -ibus ending confirms its ablative function in consonant-stem nouns.
  4. Both students could be morphologically correct, because militibus (dative/ablative plural of miles, militis) is identical in both cases; context and the semantics of dare ('to give'), which typically takes a dative of indirect object, strongly favor the dative reading, but the form itself is ambiguous. (correct answer)
Explanation: When you encounter a parsing dispute in Latin, always ask two separate questions: (1) what does the morphology allow, and (2) what does context favor? These are different questions, and conflating them is the source of this entire disagreement. Here, both students are morphologically reasonable — and that's precisely the point. The noun miles, militis is a 3rd-declension consonant-stem noun. In the plural, Latin uses -ibus as the ending for both the dative and the ablative, making these two cases identical in form. There is simply no way to distinguish dative plural from ablative plural by looking at militibus alone. Answer D captures this exactly: the form is ambiguous, but context resolves it. The verb dare ("to give") classically takes a dative of indirect object — the recipient of what is given — making the first student's reading overwhelmingly preferable in practice, even though the ending cannot confirm it on its own. Answer A is flatly wrong: -ibus is never a genitive plural ending in any declension. Genitive plural for 3rd-declension consonant-stems ends in -um. Answer B is wrong because it claims -ibus is exclusively dative in 3rd-declension nouns — it is not; -ibus serves both dative and ablative plural across the entire declension, i-stem and non-i-stem alike. Answer C is wrong on two counts: dare absolutely does govern dative indirect objects in classical prose, and "accompaniment" requires the preposition cum, not a bare ablative. The study tip: whenever you see -ibus in 3rd declension, flag it immediately as dative or ablative plural — you must use context, not morphology, to decide which.

Question 10

Which of the following Latin nouns is most likely an i-stem 3rd-declension noun, and what is the primary morphological criterion that identifies it as such?

  1. Corpus, corporis (body): the neuter nominative ending -us combined with the stem change to corpor- in the genitive is the primary signal of i-stem status, so its genitive plural would be corporum for i-stem neuters and corporum for non-i-stems alike.
  2. Consul, consulis (consul): although the genitive singular ends in -is, the nominative consul has fewer syllables than consulis, meaning it is not parisyllabic; it is therefore a non-i-stem noun, and its genitive plural is consulum, not consulium.
  3. Rex, regis (king): the stem reg- ends in a single consonant, and the difference in syllable count between nominative rex and genitive regis confirms i-stem classification, giving a genitive plural regium rather than regum.
  4. Navis, navis (ship): the nominative singular ends in -is and the genitive singular also ends in -is, giving an equal syllable count in both forms — a hallmark of parisyllabic i-stem nouns — so its genitive plural would be navium rather than navum. (correct answer)
Explanation: When tackling i-stem identification questions, focus on two diagnostic tests: parisyllabic forms (equal syllable count in nominative and genitive singular) and imparisyllabic nouns whose stem ends in two consonants. These criteria determine whether the genitive plural takes -ium (i-stem) or -um (non-i-stem). D is correct because navis, navis is a textbook parisyllabic i-stem. Both the nominative (na-vis, 2 syllables) and the genitive (na-vis, 2 syllables) have identical syllable counts — the defining hallmark of parisyllabic nouns. This parisyllabic pattern signals i-stem status, giving the genitive plural navium, not navum. A is wrong on two levels. Corpus, corporis is a non-i-stem neuter, and the explanation in the choice actually undermines itself — it admits that both i-stems and non-i-stems produce corporum, which means stem change alone cannot identify i-stem status. The genitive plural corporum here reflects the standard neuter 3rd-declension ending, not an i-stem signal. B is correct in its reasoning but misread as a wrong answer trap. Consul, consulis is genuinely non-parisyllabic (con-sul vs. con-su-lis), and its genitive plural consulum is correctly formed. The choice accurately describes a non-i-stem noun, so you should recognize it as disqualified from i-stem status, not confused as one. C is wrong because rex, regis has a stem (reg-) ending in only one consonant. Imparisyllabic nouns qualify as i-stems only when their stem ends in two consonants (like arx, arcis). The genitive plural is regum, not regium. Your study tip: memorize the two i-stem triggers — parisyllabic forms and two-consonant stems — and always check the genitive plural ending to confirm.