10 Common Grammar Mistakes & How to Avoid Them

Grammar mistakes are more common than most writers admit.

This guide walks you through the 10 most frequent grammar mistakes, why they happen, and exactly how to fix them.

Running research paper’s through Trinka’s free grammar checker before submitting is one of the simplest ways to catch what the eye misses.

Why Do Grammar Mistakes Happen So Often?

Most grammar errors are not a sign of poor intelligence. They are a sign of speed. Writers often focus on ideas and let the mechanics’ slip.

• The brain auto-corrects during reading, so errors go unnoticed on self-review.
• Rules like subject-verb agreement feel intuitive until the sentence gets complex.
• Homophones (their/there/they’re) trip up even experienced writers because spell-check does not flag them.

Mistake 1: Are You Confusing “There,” “Their,” and “They’re”?

These three words sound identical but mean completely different things. Mixing them up is one of the most spotted errors in academic and professional writing.

• “There” refers to a place: “The report is over there.”
• “Their” shows possession: “Their research was thorough.”
• “They’re” is a contraction of “they are”: “They’re presenting next week.”

Mistake 2: Do You Know When to Use “Your” vs. “You’re”?

“Your” shows ownership. “You’re” short for “you are.”

• Example (wrong): “Your going to need to revise this section.”
• Example (right): “You’re going to need to revise this section.”

Mistake 3: Is Subject-Verb Agreement Tripping You Up?

• The subject and verb in a sentence must match in number.
• A singular subject takes a singular verb.
• A plural subject takes a plural verb.
• The mistake appears most often when phrases come between the subject and verb.

Example (wrong): “The list of errors are long.”
Example (right): “The list of errors is long.”

Mistake 4: Are You Misusing Apostrophes?

Apostrophes serve two purposes: they show possession and mark contractions.

• Wrong: “The professor’s reviewed the paper’s.” (apostrophe in a simple plural)
• Right: “The professors reviewed the papers.”
• Right: “The professor’s feedback was detailed.” (possession)

Mistake 5: Do You Know the Difference Between “Who” and “Whom”?

“Who” is a subject pronoun. “Whom” is an object pronoun.

Trick: Substitute “him” or “her” in the sentence. If it fits, use “whom.” If “he” or “she” fits better, use “who.”

• “Who wrote the report?” (He wrote it. So “who” is correct.)
• “To whom should I address the letter?” (Address it to him. So “whom” is correct.)

Mistake 6: Are You Running Sentences Together Without a Break?

A run-on sentence joins two independent clauses without proper punctuation or a connecting word.

It forces the reader to do extra work to find where one idea ends and the next begins.

• Example (wrong): “The deadline is Friday the team needs to submit the draft by Thursday.”
• Example (right): “The deadline is Friday. The team needs to submit the draft by Thursday.”

Mistake 7: What Is a Comma Splice and Are You Making It?

A comma splice happens when you join two complete sentences with only a comma. It is a specific type of run-on that writers often miss because the comma makes the sentence look correct at a glance.

• Example (wrong): “She submitted the assignment; it was three days late.”
• Example (right): “She submitted the assignment, but it was three days late.”

Mistake 8: Are You Using “Less” and “Fewer” Correctly?

“Fewer” applies to things you count. “Less” applies to things you measure.

• Wrong: “There were less errors in the final draft.”
• Right: “There were fewer errors in the final draft.”
• Right: “There was less confusion after the revision.” (confusion is not countable)

Mistake 9: Do You Confuse “Affect” and “Effect”?

“Affect” is almost always a verb. “Effect” is almost always a noun.

• “The feedback affected her writing.” (verb, something acted on something else)
• “The feedback had a noticeable effect on her writing.” (noun, the result of something)

Mistake 10: Are You Ending Sentences with a Preposition When You Should Not?

Ending a sentence with a preposition (“with,” “on,” “at,” “for”) is accepted in casual writing.

In formal academic or professional work, it often needs restructuring.

• Casual: “This is the rule I was referring to.”
• Formal: “This is the rule to which I was referring.”

How Do You Build a Habit of Catching These Mistakes?

The fastest way to reduce grammar errors is to slow down during the review stage.

Most writers spend 90% of their time writing and almost none editing. That ratio needs to change.

• Leave a gap between writing and editing so you read what is actually on the page.
• Use Trinka’s free grammar checker as your final pass. It is built specifically for academic and professional writing and flags subject-verb disagreements, misused articles, and punctuation errors that generic tools miss.