Grammar Mistakes Students Make (And How AI Fixes Them)

The 8 most common grammar errors in student essays — with before and after examples showing exactly how AI grammar tools fix them.

Grammar errors in student essays are not just cosmetic problems. Research on academic marking consistently shows that poor grammar reduces perceived credibility — even when the underlying ideas are strong. Examiners form impressions quickly, and a dense paragraph of comma splices and subject-verb disagreements signals carelessness, not intelligence.

The good news: modern AI grammar tools catch these errors reliably, explain them clearly, and suggest corrections that preserve your original voice.

The 8 Most Common Student Grammar Errors

The Comma Splice What it is: Joining two independent clauses with just a comma.

Example with error: "The economy grew rapidly in the 1990s, this led to significant social change."

Fixed: "The economy grew rapidly in the 1990s. This led to significant social change." (or use a semicolon, or add "which")

AI grammar checkers flag comma splices reliably and suggest three options: period, semicolon, or coordinating conjunction.

Subject-Verb Disagreement What it is: The subject and verb do not match in number.

Example with error: "The results of the study shows a significant correlation."

Fixed: "The results of the study show a significant correlation."

The trick: the subject is "results" (plural), not "study." Students often match the verb to the nearest noun, which is wrong.

Dangling Modifiers What it is: A descriptive phrase that does not clearly attach to the right noun.

Example with error: "Having studied all night, the exam felt manageable."

Fixed: "Having studied all night, I found the exam manageable."

The original implies the exam did the studying. AI grammar tools catch these surprisingly well.

Inconsistent Tense What it is: Switching between past and present tense without reason.

Example with error: "The researcher collected data and then analyses it carefully."

Fixed: "The researcher collected data and then analysed it carefully."

Passive Voice Overuse What it is: Using passive construction when active would be clearer.

Passive: "The experiment was conducted by the team." Active: "The team conducted the experiment."

Passive voice is not always wrong — it is appropriate in scientific writing when the action matters more than the actor. But overusing it makes prose feel bureaucratic and distant.

Apostrophe Errors What it is: Confusing its/it's, their/they're, or adding apostrophes to plurals.

Example with error: "Each student must submit their's before the deadline. It's results will be reviewed."

Fixed: "Each student must submit theirs before the deadline. Its results will be reviewed."

Run-On Sentences What it is: Multiple clauses strung together without proper punctuation.

Example with error: "AI has transformed education students can now access personalised learning tools that adapt to their individual needs and this has led to improved outcomes across different demographic groups."

Fixed: Break into 2–3 sentences, each with a clear subject and single main idea.

Vague Pronoun Reference What it is: A pronoun that could refer to multiple nouns.

Example with error: "The teacher told the student that he needed to revise his essay."

Fixed: Rewrite to clarify: "The teacher told the student, 'You need to revise your essay.'"

How to Use AI Grammar Checker Effectively

Navigate to Grammar Checker and paste your completed essay draft. Do not use it while writing — editing and writing simultaneously breaks creative flow.

Review each suggestion individually. Not every AI suggestion improves your writing; sometimes the original phrasing is better for stylistic reasons. Accept corrections that are clearly errors; use your judgment on style suggestions.

The Right Order for Editing

Structural edit — Does your argument flow logically? (Do this yourself) Clarity edit — Is each sentence saying exactly what you mean? (Do this yourself) Grammar check — Are there technical errors? (Use AI Grammar Checker here) Final read — Does it sound like you? (Do this yourself)

Running grammar checks before you have finished thinking about content wastes time, because structural changes will introduce new errors anyway.

Run your next essay through Grammar Checker before submission.