Grammar Mistakes in Student Essays: 12 Common Errors and Fixes

Markers rarely mark down an essay for one comma. But repeated grammar errors add up. They slow the reader, blur your meaning, and signal that the work was not checked, which can pull down the marks for clarity and presentation even when your ideas are strong.

The encouraging news is that most student grammar problems fall into a short list of patterns. Learn to spot these 12 and you will fix a large share of the errors in your own writing. Each one below includes a before-and-after example and a quick test.

1. Comma Splices and Run-On Sentences

A comma splice joins two complete sentences with only a comma. A run-on joins them with nothing at all.

Before: The experiment failed, the results were inconclusive.

After: The experiment failed, and the results were inconclusive.

Fix it: Use a period, a semicolon, or a comma plus a conjunction such as and, but, or so. Test: if you can put a period where the comma is and both halves stand alone, you need something stronger than a comma.

2. Sentence Fragments

A fragment is missing a subject or a verb, or it begins with a word like because or although and never finishes the thought.

Before: Although the sample was small.

After: Although the sample was small, the results were consistent.

Fix it: Attach the fragment to a nearby sentence or complete the thought. Fragments are acceptable in some creative writing but rarely in academic essays.

3. Subject-Verb Agreement Errors

The verb must match its subject in number, and long sentences make mistakes easy.

Before: The results of the survey shows a clear trend.

After: The results of the survey show a clear trend.

Fix it: Find the real subject ("results," not "survey") and ignore the phrase in between. Collective nouns and phrases like "a number of" and "the number of" cause the most trouble: "a number of students were" but "the number of students was."

4. Misplaced and Dangling Modifiers

A modifier must sit next to the word it describes. A dangling modifier describes something that is not in the sentence.

Before: Walking to the lecture, the rain started.

After: Walking to the lecture, I got caught in the rain.

Fix it: Ask who or what is performing the action in the opening phrase, and make that the subject of the main clause.

5. Pronoun Problems

Pronouns such as it, this, they, and which must clearly refer to one noun.

Before: The study compared the interviews with the questionnaires, and it was more reliable.

After: The study compared the interviews with the questionnaires, and the questionnaires were more reliable.

Fix it: If the reader could reasonably point to two nouns, replace the pronoun with the noun. Be especially careful with a stand-alone "this." Write "this finding" or "this approach" instead.

6. Inconsistent Verb Tense

Switching tense without reason confuses the timeline.

Before: Smith (2019) found that participants improved, and then she says the effect lasts a year.

After: Smith (2019) found that participants improved, and she argued that the effect lasts a year.

Fix it: Choose a tense for each part of the essay and keep it. In many fields, studies are reported in the past tense and established facts in the present.

7. Apostrophe Errors

Apostrophes show possession or contraction, not plurals.

Before: The participant's completed the surveys, and its results are clear.

After: The participants completed the surveys, and the results are clear.

Fix it: Remember that "it's" means "it is" or "it has," while "its" is possessive. Avoid contractions in formal essays. Plural nouns never take an apostrophe, but possessive plurals go after the s: "the students' work."

8. Commonly Confused Words

Spell check will not catch words that are spelled correctly but used wrongly.

Before: The new policy will effect student behavior.

After: The new policy will affect student behavior.

The usual offenders:

• affect (verb: to influence) and effect (noun: a result)
• then (time) and than (comparison)
• their, there, and they're
• its and it's
• principal (main, or a school head) and principle (a rule)
• fewer (countable) and less (uncountable)

Fix it: Keep a personal list of the pairs you confuse and search for them in your final read.

9. Faulty Parallelism

Items in a list or comparison should have the same grammatical form.

Before: The course teaches students to research, writing clearly, and how to cite sources.

After: The course teaches students to research, write clearly, and cite sources.

Fix it: Check that each item in a list begins with the same type of word, such as all verbs or all nouns.

10. Unnecessary or Missing Commas

Commas that interrupt a sentence's flow, and missing ones that cause confusion, are among the most common mark-downs.

Before: After the surveys were collected the researchers, began the analysis.

After: After the surveys were collected, the researchers began the analysis.

Fix it: Put a comma after a long introductory phrase, around non-essential information, and before a conjunction that joins two full sentences. Do not put a comma between a subject and its verb.

11. Wordiness and Passive Overuse

Padding hides your point, and heavy use of passive voice can make your claims sound evasive.

Before: It can be seen from the data that there is a significant increase in the amount of participants who responded.

After: The data show a significant increase in participants who responded.

Fix it: Cut empty phrases like "it can be seen that" and "in order to." Passive voice is not an error, and it is common in scientific writing, but if every sentence uses it, check whether active voice would be clearer.

12. Inconsistent Spelling Conventions

Mixing American and British spelling within one essay looks careless.

Before: One paragraph uses "color" and the next uses the British form of the same word.

After: Every paragraph uses "color" (or every paragraph uses the British form, if your course requires it).

Fix it: Check which variety your institution or style guide expects, and use it throughout. Set your word processor's language to match so it flags the other.

How to Find These Errors in Your Own Essay

Grammar errors are hard to see in your own writing, so use a method instead of a general reread:

1. Read the essay aloud, slowly. Awkward spots usually mean a grammar problem.
2. Read the last sentence first and work backward, one sentence at a time.
3. Search for your known weak spots: "its," "affect," "this," and commas before "which."
4. Use a grammar checker as a first filter, but not as the final judge. Automated tools miss context and sometimes suggest changes that introduce new errors.

For a complete final-pass routine, use our essay proofreading checklist.

When Grammar Is a Bigger Challenge

If you write in English as a second language, grammar patterns from your first language may appear in your essays in ways a checker does not catch. In that case, a human editor who can explain patterns, not just correct them, is especially valuable. We explain how that works in ESL and Second-Language Academic Editing. Grammar is only one part of an essay's quality, so also review 10 Essay Writing Mistakes That Cost You Marks for the structural and content errors that grammar alone cannot fix.

Summary

Most grammar problems come from a short list: splices, fragments, agreement, modifiers, pronouns, tense, apostrophes, confused words, parallelism, commas, wordiness, and inconsistent spelling. Learn the quick tests, read aloud, and search for your own patterns, and your writing will improve quickly.

Where to Go Next

• Want a human editor to catch what you miss? See our editing and proofreading service.
• Learn how editing levels differ: Proofreading vs Editing.
• Build your skills with academic coaching, and see the pricing.

From choosing a topic to polishing a final draft, CampusScribe's subject-specialist editors and coaches help with dissertations, capstone projects, presentations, and more - whether you need topic guidance, structural feedback, or a final edit before your deadline.