Recognize sentence boundary errors (run-ons, fragments) (as tested) - TACHS Written Expression

SECTION 1

Why Sentence Boundaries Matter

Have you ever read a text message that was so long and jumbled you couldn't figure out what the person meant? That's basically what happens when writers ignore sentence boundaries (the places where one complete thought ends and another begins). For thousands of years, people have been developing rules to keep writing clear.

Ancient writers didn't even use spaces between words! Over time, punctuation marks like the period, comma, and semicolon were invented to help readers know when to pause and when a thought was done. Let's look at how these rules developed.

200 BCE • Early Punctuation Invented

A librarian in ancient Greece named Aristophanes created dots placed at different heights to show readers where to pause. This was the birth of punctuation.

700s CE • Spaces Between Words

Irish monks began adding spaces between words when copying Latin texts. Before this, all words ran together with no gaps at all.

1400s • The Printing Press Standardizes Rules

When Gutenberg invented the printing press, printers needed consistent rules for periods, commas, and capital letters. Sentence boundaries became much more standard.

1800s • Grammar Textbooks for Schools

Schools began teaching students formal grammar rules, including how to write complete sentences and avoid fragments and run-ons.

Today • Tested on Standardized Exams

Exams like the TACHS test your ability to spot sentence boundary errors. These skills help you communicate clearly in school, at work, and in everyday life.

So, the big question is: how do you know when a sentence is complete? And how do you spot errors when sentences are smashed together or cut short? That's exactly what this lesson will teach you.

SECTION 2

Core Principles: What Makes a Sentence Complete?

Before you can spot errors, you need to understand what a correct sentence looks like. A complete sentence (also called an independent clause) has three things: a subject, a verb, and a complete thought. When any of these parts is missing, or when two complete sentences are glued together without proper punctuation, you've got a sentence boundary error.

Complete Sentence

Has a subject (who or what), a verb (action or state of being), and expresses a complete thought. Example: "The dog barked loudly."

Fragment

A piece of a sentence that is missing a subject, a verb, or a complete thought. It looks like a sentence (starts with a capital, ends with a period) but isn't one. Example: "Running down the street."

Run-on Sentence

Two or more complete sentences joined without correct punctuation or a connecting word. Example: "I like pizza it is my favorite food."

Comma Splice

A special type of run-on where two complete sentences are joined only by a comma. Example: "I studied hard, I passed the test." A comma alone is not strong enough to connect two full sentences.

✦

KEY TAKEAWAY

Think of a sentence like a train car. A complete sentence is a car with an engine (verb), a conductor (subject), and a destination (complete thought). A fragment is a car missing its engine — it can't go anywhere on its own. A run-on is two trains crashing into each other because nobody put a signal (punctuation) between them.

SECTION 3

Seeing Sentence Boundaries

The diagram below shows you the three sentence structures you'll need to recognize on the TACHS. Study the color-coded labels to see what's present and what's missing in each type.

Green shows a correct complete sentence. Pink shows a fragment — notice the missing pieces. Yellow shows a run-on where two complete sentences crash together without proper punctuation.

Notice in the diagram that the green box has all three parts: a subject ("The cat"), a verb ("sat"), and a complete thought. The pink box is missing a subject and a full verb — "Running" is just part of a verb without a helper. The yellow box has two perfectly good sentences, but nothing proper sits between them.

SECTION 4

How to Identify Errors Step by Step

On the TACHS, you'll read a sentence (or group of words) and decide if there is an error. Here is a step-by-step method you can use every single time. Think of it as a checklist.

The 3-Question Test

Question 1: Does it have a subject? Look for the person, place, thing, or idea that the sentence is about. If there is no subject, it is a fragment.
Question 2: Does it have a complete verb? A complete verb shows action or a state of being. Words ending in "-ing" by themselves (like "running") are NOT complete verbs. They need a helper (like "is running"). No complete verb means it is a fragment.
Question 3: Does it express a complete thought? If it starts with a word like "because," "when," "although," or "if" and doesn't finish the idea, it is a fragment even if it has a subject and verb.

Spotting Run-ons and Comma Splices

If the group of words passes all three questions, check one more thing: is there more than one complete thought? If yes, make sure they are joined correctly. Two complete thoughts can be joined by (1) a period, (2) a semicolon, or (3) a comma plus a coordinating conjunction (for, and, nor, but, or, yet, so — remember the acronym FANBOYS). If none of these are present, it's a run-on.

💡

TACHS Tip

On the TACHS, you won't fix the error — you just need to find it. Look at the answer choices and ask yourself: "Does this group of words pass the 3-Question Test?" If it fails any question, that answer choice contains the error.

SECTION 5

Types of Fragments and Run-ons

Not all fragments look the same, and not all run-ons are obvious. The TACHS tests several specific types. Let's break them down so you know exactly what to watch for.

This tree diagram shows the two main families of sentence boundary errors. On the left (pink), three types of fragments. On the right (yellow), two types of run-ons. Each box includes an example and a hint question to help you identify the error.

Error Type	What's Wrong	Example	Quick Fix
No-Subject Fragment	Missing who or what	"Jumped over the fence."	Add a subject: "The dog jumped over the fence."
No-Verb Fragment	Missing action	"The bright blue sky."	Add a verb: "The bright blue sky sparkled."
Dependent Clause Fragment	Starts with because, when, if, although, etc. — incomplete thought	"When the bell rang."	Finish the thought: "When the bell rang, we left."
Fused Run-on	Two sentences with no punctuation between them	"I ran fast she ran faster."	Add a period or semicolon: "I ran fast. She ran faster."
Comma Splice	Two sentences joined with only a comma	"It rained all day, we stayed inside."	Use a period, semicolon, or add FANBOYS: "It rained all day, so we stayed inside."

Common sentence boundary errors tested on the TACHS

SECTION 6

Worked Example: Finding the Error

Let's walk through a TACHS-style question together. On the test, you'll read a set of sentences and pick the one that contains an error — or choose "No mistake" if they're all correct.

ℹ

SAMPLE QUESTION

Which underlined section contains an error? (A) Maria studied all night for the science exam. (B) Because she wanted to earn an A on the test. (C) Her hard work paid off in the end. (D) No mistake.

Step-by-Step Solution

1
Step 1 — Check Choice (A)

"Maria studied all night for the science exam." Subject? Yes — Maria. Verb? Yes — studied. Complete thought? Yes — we know what happened. This is a complete sentence. No error here.
2
Step 2 — Check Choice (B)

"Because she wanted to earn an A on the test." Subject? Yes — she. Verb? Yes — wanted. Complete thought? No! The word "Because" makes this a dependent clause. It leaves you hanging — because she wanted an A … what happened? This is a fragment.

ERROR FOUND — Fragment (dependent clause standing alone)
3
Step 3 — Check Choice (C) to be sure

"Her hard work paid off in the end." Subject? Yes — her hard work. Verb? Yes — paid off. Complete thought? Yes. This is correct.
4
Step 4 — Select the answer

Since choice (B) is a fragment, the correct answer is (B). We would not choose (D) "No mistake" because there IS a mistake.

Answer: (B)

SECTION 7

Common Traps and How to Avoid Them

The TACHS is tricky because some errors look fine at first glance. Here are the most common traps students fall into and how to beat them.

Trap	Why It Tricks You	How to Beat It
Long fragments	A fragment can be very long, which makes it look like a real sentence. "The tall boy standing near the basketball court with his friends after school."	Length doesn't matter. Ask: where is the verb? "Standing" is not a complete verb here — it needs "was standing."
Short complete sentences	A short sentence like "She ran." may look too small to be correct, so students mark it as a fragment.	Check the 3-Question Test. "She" = subject, "ran" = verb, complete thought = yes. It's fine!
"Because" and "Although" fragments	These have a subject and verb, so students think they're complete. "Although the weather was nice."	Words like because, although, when, and if create dependent clauses. They need an independent clause to finish the thought.
Comma + "however" splice	Students think words like "however" and "therefore" work like FANBOYS. "I was tired, however I kept going."	Words like however, therefore, and furthermore are NOT FANBOYS. Use a semicolon before them: "I was tired; however, I kept going."

Common TACHS traps with sentence boundary errors

✦

KEY TAKEAWAY

Think of subordinating conjunctions (because, when, although, if, since) like leashes on a dog. A dog on a leash can't go off on its own — it needs to be attached to something. A clause that starts with one of these words can't stand alone either. It needs to be attached to a complete sentence.

SECTION 8

Connecting to Stronger Writing

Spotting fragments and run-ons on a test is great, but this skill also makes you a better writer. In high school and beyond, you'll use more complex sentence structures. Understanding boundaries now will help you handle those with confidence.

Skill Now (TACHS Level)	Skill Later (High School & Beyond)
Identify fragments and run-ons	Fix fragments and run-ons in your own essays and reports
Recognize FANBOYS conjunctions	Use compound and complex sentences with variety and style
Spot dependent clauses standing alone	Combine clauses on purpose for emphasis and flow in persuasive writing
Know that a comma alone can't join two sentences	Use semicolons, colons, and dashes to create advanced sentence patterns

How TACHS skills connect to future writing

Some professional writers use fragments on purpose for dramatic effect, like "Silence. Nothing." But that's a style choice made by experienced writers. On the TACHS, always treat fragments as errors unless the question specifically says otherwise.

SECTION 9

Practice Problems

Try these five problems. For each one, decide if there is a fragment, a run-on (including comma splices), or no error. Then check your answer below.

Question 1: CONCEPTUAL

Which of the following is a sentence fragment? (A) The stars twinkled brightly. (B) After the storm ended last night. (C) We ate dinner together. (D) No mistake.

Show answer

The answer is (B). "After the storm ended last night" begins with the subordinating conjunction "after," which makes it a dependent clause. Even though it has a subject ("the storm") and a verb ("ended"), it does not express a complete thought. You're left wondering: what happened after the storm ended? Choices (A) and (C) are both complete sentences with subjects, verbs, and complete thoughts.

Question 2: BASIC IDENTIFICATION

Identify the error in the following: (A) My brother plays soccer every Saturday. (B) He is the best player on his team. (C) Scored three goals in the last game. (D) No mistake.

Show answer

The answer is (C). "Scored three goals in the last game" is a fragment because it is missing a subject. Who scored? We can guess it's "he" from context, but the sentence as written doesn't tell us. Choice (A) has the subject "My brother" and verb "plays." Choice (B) has the subject "He" and verb "is." Both are complete.

Question 3: INTERMEDIATE

Which sentence contains a comma splice? (A) The movie was exciting, but it was too long. (B) We arrived early, we got the best seats. (C) Although it rained, the game continued. (D) No mistake.

Show answer

The answer is (B). "We arrived early, we got the best seats" contains a comma splice. Both "We arrived early" and "we got the best seats" are complete sentences. A comma alone cannot join them. You'd need a period, semicolon, or a comma plus a FANBOYS conjunction (like "and" or "so"). Choice (A) is correct because it uses a comma plus "but" (a FANBOYS word). Choice (C) is correct because the dependent clause "Although it rained" is properly attached to the independent clause "the game continued."

Question 4: APPLIED

Read the paragraph below. Which underlined portion contains a sentence boundary error? (A) The school science fair is next Friday. (B) Students have been working on their projects for weeks. (C) The gymnasium decorated with colorful banners and posters. (D) No mistake.

Show answer

The answer is (C). "The gymnasium decorated with colorful banners and posters" is a fragment because it is missing a complete verb. "Decorated" might look like a verb, but here it acts as a past participle describing the gymnasium — like saying "The decorated gymnasium." To fix it, you'd need a helping verb: "The gymnasium was decorated with colorful banners and posters." This is a tricky type of fragment because it's long and looks complete at first glance.

Question 5: CRITICAL THINKING

A student writes: "I wanted to go to the park, however, the rain started before I could leave the house I decided to read a book instead." How many sentence boundary errors does this passage contain? Describe each one.

Show answer

This passage contains two sentence boundary errors. First, "I wanted to go to the park, however, the rain started" is a comma splice. "However" is not a FANBOYS conjunction, so a comma before it cannot join two independent clauses. It should be: "I wanted to go to the park; however, the rain started." Second, "the rain started before I could leave the house I decided to read a book instead" is a fused run-on. Two complete thoughts — "the rain started before I could leave the house" and "I decided to read a book instead" — are jammed together with no punctuation. A period or semicolon is needed between "house" and "I."

SUMMARY

Lesson Summary

Summary

A complete sentence needs three things: a subject, a complete verb, and a complete thought. A fragment is missing one or more of these parts. Watch for sentences that lack a subject, have no full verb (just an "-ing" word alone), or start with words like because, when, although, or if without finishing the thought.

A run-on sentence jams two complete thoughts together without proper punctuation. A comma splice is a run-on that uses only a comma — which isn't strong enough. To correctly join two sentences, use a period, a semicolon, or a comma plus a FANBOYS conjunction (for, and, nor, but, or, yet, so). On the TACHS, use the 3-Question Test to check every answer choice quickly and confidently.