Why Sentence Errors Matter
Every day, people write emails, job applications, reports, and school assignments. When a sentence is missing a piece or runs together without a break, readers can get confused—they have to stop and re-read to figure out what you meant. Learning to spot and fix these problems makes your writing clearer and more professional. The three most common sentence errors are run-on sentences, sentence fragments, and tense shifts. This lesson will help you recognize and fix all three.
Run-ons, fragments, and tense shifts are among the most common writing errors people make—not just beginners, but experienced writers too. A large study of student writing found that run-ons and comma splices ranked among the top five errors in thousands of papers. The good news is that once you know what to look for, these errors are very fixable. This lesson gives you a clear, step-by-step way to find and correct them.
Core Principles & Definitions
Before you can fix sentence errors, you need to know what makes a sentence correct. A complete sentence has a subject (who or what the sentence is about), a verb (the action or state), and it expresses a complete thought. When a sentence is missing one of these parts—or when two complete sentences are stuck together without the right punctuation—you get a sentence error. The three types below cover most of the sentence-level problems you will see in everyday writing.
Run-On Sentence
Sentence Fragment
Tense Shift
Complete vs. Incomplete Sentence
Visual Explanation — Anatomy of a Sentence
The chart above gives you three questions to ask about any sentence. First, does the group of words have both a subject and a verb? If one is missing, it is a fragment. Second, if the sentence contains more than one complete thought, are those thoughts joined by a period, semicolon, or a comma plus a joining word? If not, you have a run-on or comma splice. Third, do the verbs stay in the same tense unless there is a real reason to change? If they jump around, you have a tense shift. Using these three questions turns error-checking into a simple, step-by-step habit.
How Each Error Works — Examples and Fixes
Run-On Sentences & Comma Splices
A fused sentence happens when two complete sentences are placed next to each other with nothing between them: The experiment failed the researchers revised their hypothesis. A comma splice is similar, but the writer puts a comma between them—which is not enough: The experiment failed, the researchers revised their hypothesis. Both errors leave the reader unsure where one idea ends and the next begins. There are four easy ways to fix them: (1) add a period and start a new sentence; (2) add a semicolon; (3) add a comma followed by a joining word (for, and, nor, but, or, yet, so—known as FANBOYS); or (4) use a connecting word like because, although, or when to show how the ideas are related.
Sentence Fragments
Fragments come in a few common forms. A fragment can start with a word like because, although, or when but have nothing else to complete the thought: Because the funding was cut. This leaves the reader waiting—because what? A fragment can also be missing a verb: Running through the data set for hours. (Who was running? There is no subject doing a real action here.) A fragment can also be missing a subject: And then decided to leave. To fix a fragment, you can attach it to a nearby sentence, add the missing subject or verb, or remove the opening connecting word.
Tense Shifts
Verb tense tells your reader when something happens. A tense shift becomes an error when the tense changes for no clear reason. Consider: The author argues that social media was harmful. The verb argues is present tense, but was is past tense. Since the author's argument is still in print and still being discussed, both verbs should be present tense: The author argues that social media is harmful. Sometimes, however, switching tenses is correct—when events actually happen at different times: The bridge collapsed in 1940, and engineers now study the failure to improve modern designs. Here the switch from past (collapsed) to present (study) makes sense because the two actions really do happen at different times.
Error Classification & Repair Strategies
| Error Type | Quick Check | How to Fix It |
|---|---|---|
| Fused sentence | Can you find two complete sentences with nothing between them? | Period; semicolon; comma + FANBOYS; connecting word |
| Comma splice | Is a comma the only thing separating two complete sentences? | Replace comma with period or semicolon; add a joining word after the comma |
| Fragment (incomplete thought) | Does it start with a word like because, although, or when and have nothing attached? | Attach to the sentence before or after; remove the opening connecting word |
| Fragment (missing subject or verb) | Is there a subject? Is there a main action verb? If either is missing, it is a fragment. | Add the missing subject or verb; attach to a nearby sentence |
| Tense shift (unneeded) | Underline every verb. Does the tense change without a real change in time? | Change the odd verb(s) to match the tense used throughout the rest of the passage |
Worked Example — Revising a Flawed Paragraph
Below is a short paragraph with several sentence errors. We will walk through finding and fixing each one, step by step, using the questions from Section 3.
Strengths, Limitations & Common Pitfalls
Knowing how to correct run-ons, fragments, and tense shifts will make your writing much cleaner. But it also helps to know when the rules can be bent—and what traps to watch out for.
| Strength | Limitation / Pitfall |
|---|---|
| Fixing run-ons makes it clear where one idea ends and another begins, so readers follow your writing more easily. | If you break every sentence into very short pieces, your writing can feel choppy and hard to read. Some ideas belong together. |
| Fixing fragments ensures every sentence says something complete, which makes your writing sound more professional. | Sometimes writers use a short fragment on purpose for effect (e.g., 'Absolutely not.'). In creative writing, this can work well—but in school or work writing, it is usually best to avoid fragments. |
| Keeping verb tenses consistent helps readers follow the time order of events without getting confused. | Some tense changes are needed and correct—for example, when events really do happen at different times. Not every tense change is an error. |
| Using a checklist of questions gives you a reliable way to edit your own writing, even under time pressure. | Fixing only grammar without thinking about your reader or purpose can produce writing that is technically correct but still hard to understand. |
Connection to Stronger Writing & Revision
The skills you learned in this lesson are the building blocks of stronger writing overall. Once you can spot and fix sentence errors, you are ready to work on bigger skills: combining short sentences (joining two related sentences into one more interesting sentence), parallel structure (making items in a list sound balanced and even), and paragraph flow (using transitions and clear topic sentences to guide the reader through your ideas from one paragraph to the next).
| This Lesson (Sentence Errors) | Next-Level Writing Skills |
|---|---|
| Fix run-ons by separating or correctly joining sentences | Combining sentences: join related ideas using connecting words and phrases to add variety and show how ideas relate |
| Fix fragments by adding missing parts | Purposeful short sentences: use brief sentences for emphasis or effect in the right kind of writing |
| Fix tense shifts by keeping verb tenses consistent | Advanced verb use: learn to use different verb forms to show when things happened, how long they lasted, or whether they are certain or uncertain |
| Use a checklist to review individual sentences | Big-picture revision: look at whether your whole piece is organized well, whether your paragraphs connect, and whether your ideas are clear to your reader |
Writing teachers and researchers agree that fixing sentence errors is important—but it is just one part of good writing. After you fix the errors, you can step back and ask bigger questions: Do my sentences work together to build a clear point? Does each paragraph stay on topic? Does my writing say what I mean in a way my reader will understand? Fixing errors in this lesson frees up your attention so you can focus on those bigger questions as you keep growing as a writer.
Practice Problems
Lesson Summary
This lesson covered the three most common sentence errors in everyday writing. A run-on sentence (including fused sentences and comma splices) happens when two complete sentences are joined without the right punctuation or connecting word. The four fixes are a period, a semicolon, a comma plus a FANBOYS joining word, or a connecting word that shows how the ideas relate. A sentence fragment is missing a subject, a verb, or a complete thought; the fix is to add the missing piece or attach the fragment to a nearby sentence. An unneeded tense shift happens when the verb tense changes for no real reason; the fix is to change the odd verb to match the tense used in the rest of the passage.
The three-question checklist from Section 3 gives you a simple, repeatable way to check your sentences: Does it have a subject and a verb? Are complete sentences joined correctly? Do the verb tenses stay consistent? Using these questions as a habit will improve your editing. Once you have mastered sentence errors, you will be ready to work on bigger skills like combining sentences, parallel structure, and paragraph flow. Remember: the goal is not to follow rules without thinking, but to make every sentence choice a decision, not an accident.