Common Grammar Mistakes and How to Fix Them

Good grammar matters whether you're writing a college essay, a business email, or a professional report. Even experienced writers make common grammar mistakes, and many of them are easy to miss because they don't trigger spell-check. Knowing which errors to watch for is the first step toward cleaner, more credible writing.

This guide covers the most common grammar mistakes in English, explains why they happen, and shows you how to fix them. For writing that needs to be error-free, a professional editor can catch what grammar tools miss.

The Most Common Grammar Mistakes in English

The most common grammar mistakes fall into a few recurring categories: confusing homophones (words that sound the same but mean different things), incorrect apostrophe use, inconsistent capitalization, subject-verb and pronoun agreement, punctuation problems, and a small set of commonly confused word pairs. Here's a breakdown of each.

They're, Their, and There

This is one of the most frequently made grammar mistakes in everyday writing, and it's easy to see why. All three words sound identical, which means spell-check won't flag the wrong one. Understanding the difference comes down to knowing what each word does in a sentence.

• They're is a contraction of "they are."
• Their is a possessive pronoun, like "his" or "hers," indicating that something belongs to a group of people.
• There is most often an adverb indicating a place, or a pronoun used to introduce a clause.

Here's an example sentence with all three words used incorrectly:

Incorrect:Their going to leave there car over they're by the curb.

To test whether you've used the right word, substitute a longer phrase in its place. If the sentence still makes sense, you have the right one. Here's the corrected version with the substitution test applied:

Correct:They're [they are] going to leave their [belonging to them] car over there [in that place] by the curb.

This substitution test works for all homophones. If replacing the word with its expanded meaning produces a logical sentence, you have the correct version.

You're and Your

You're and your are another pair of homophones that are frequently swapped by mistake. The distinction is straightforward once you know it.

• You're is a contraction of "you are."
• Your is a possessive adjective describing something that belongs to the person you're addressing.

Here's an example with both words swapped:

Incorrect:Your sure you're order isn't ready?

Apply the same substitution test to check:

Correct:You're [you are] sure your [belonging to you] order isn't ready?

With the correct words in place, the sentence reads naturally. Whenever you write you're, try replacing it with "you are." If the sentence still makes sense, you have the right word.

It's and Its

It's and its are written almost identically and pronounced exactly the same, which makes this one of the most common punctuation-related grammar mistakes. The apostrophe makes all the difference. For a deeper breakdown, see our article on it's vs. its.

• It's is a contraction of "it is" or "it has."
• Its is a possessive pronoun indicating that something belongs to a thing or animal.

Here's a common example of the mistake:

Incorrect:Its quarter to three.

To check, expand the contraction and read the sentence aloud:

Correct:It is quarter to three.

That reads correctly, so it's is the right choice here. The possessive its would only be correct in a sentence like "The clock showed its age."

Misplaced Apostrophes

Apostrophe errors are among the most common grammar mistakes in English. They appear in three main situations: contractions (it's, you're, they're), possessives (the editor's notes, the company's policy), and incorrectly in plurals, where no apostrophe is needed at all.

The most common apostrophe mistake is adding one to form a simple plural. For example:

• Incorrect: The report's are ready.
• Correct: The reports are ready.

Apostrophes are never used to form ordinary plurals. They indicate either a contraction or possession. When in doubt, ask whether the word is shortening two words into one or showing that something belongs to someone. If neither applies, no apostrophe is needed.

For singular possessives, the apostrophe comes before the s (the editor's feedback). For plural nouns that already end in s, the apostrophe comes after (the editors' feedback).

Inconsistent Capitalization

Inconsistent capitalization is a common grammar mistake that signals a lack of attention to detail. It most often appears in titles and headings, where writers are unsure whether to capitalize every word or only the major ones.

Both of the following conventions are correct, but they can't be mixed within the same title:

• Capitalize all major words: Why I'm Afraid of the Dark
• Capitalize every word: Why I'm Afraid Of The Dark

The mistake occurs when a writer uses both conventions in the same title or heading, capitalizing some minor words like "of" and "the" while leaving others lowercase. Choose one convention and apply it consistently throughout your document.

Subject-Verb Agreement Errors

Subject-verb agreement errors occur when the verb in a sentence doesn't match the number of the subject. A singular subject takes a singular verb, and a plural subject takes a plural verb. The mistake most often happens when a phrase or clause separates the subject from the verb, making it easy to lose track of which noun the verb should agree with.

• Incorrect: The quality of the reports are declining.
• Correct: The quality of the reports is declining.

In this example, the subject is "quality," not "reports." Identifying the true subject before choosing a verb form prevents this common error.

Another example involves "list of":

• Incorrect: The list of requirements were submitted on time.
• Correct: The list of requirements was submitted on time.

Collective nouns, indefinite pronouns, and compound subjects are other common sources of subject-verb agreement errors. When in doubt, identify the true subject of the sentence before choosing the verb form.

Pronoun-Antecedent Disagreement

A pronoun must agree in number and gender with the noun it refers to (its antecedent). This sounds straightforward, but errors are common, particularly with singular indefinite pronouns and with the use of "they" as a singular pronoun.

• Traditional formal: Every student must submit his or her assignment by Friday.
• Also acceptable: All students must submit their assignments by Friday.

The use of "they" as a gender-neutral singular pronoun is increasingly accepted in both general and professional writing, including by major style guides like APA and the Chicago Manual of Style. However, in formal academic writing, it's worth checking the style guide requirements of your institution or target journal before using it.

Comma Splices and Run-On Sentences

A comma splice occurs when two independent clauses are joined with only a comma, creating a run-on sentence. Each independent clause contains a subject and a verb and can stand on its own as a complete sentence. Joining them with a comma alone is a grammatical error.

• Incorrect: The deadline is tomorrow, I haven't started yet.
• Correct: The deadline is tomorrow, but I haven't started yet.
• Also correct: The deadline is tomorrow. I haven't started yet.

To fix a comma splice, either add a coordinating conjunction (and, but, so, yet, or, nor, for) after the comma, replace the comma with a semicolon, or split the clauses into two separate sentences.

A related error is the sentence fragment, a group of words that doesn't form a complete sentence because it's missing a subject, a verb, or both. Both run-on sentences and fragments make writing harder to follow and signal a lack of control over sentence structure. For a deeper dive on comma rules, see our articles on eight comma tips for writers and how to use commas correctly.

Dangling and Misplaced Modifiers

A modifier is a word or phrase that describes something else in the sentence. When the modifier is placed too far from what it describes, or when what it describes is missing from the sentence entirely, the result is a confusing or unintentionally funny error.

• Dangling modifier: Having reviewed the data, the conclusions were clear.
• Corrected: Having reviewed the data, the researchers found the conclusions were clear.

In the first example, "having reviewed the data" appears to modify "the conclusions," which can't review data. The subject doing the reviewing (the researchers) is missing. Always make sure the modifier is placed next to the word it's actually describing.

Who and Whom

"Who" and "whom" trip up even confident writers. The rule is consistent: use "who" when it refers to the subject of a clause, and "whom" when it refers to the object.

A simple test: substitute "he" or "him" in the clause. If "he" fits, use "who." If "him" fits, use "whom."

• Who submitted the report? He submitted the report. So "who" is correct.
• To whom was the report sent? The report was sent to him. So "whom" is correct.

Inconsistent Verb Tense

Shifting between verb tenses within a piece of writing without a good reason is a common problem, particularly in academic writing and long-form professional documents. Decide on the appropriate tense for each section of your document and apply it consistently throughout.

In academic writing, the literature review is typically written in past tense (Smith argued, Jones found), while the present tense is used for statements of established fact (research shows, the data indicate). Whatever conventions apply in your field or style guide, apply them consistently from start to finish.

Less vs. Fewer, Affect vs. Effect, and Other Confused Pairs

Several word pairs cause consistent confusion in professional writing. Here are the ones that appear most often:

• Less vs. fewer. Use "fewer" for countable nouns (fewer mistakes, fewer pages) and "less" for uncountable nouns (less time, less effort).
• Affect vs. effect. "Affect" is usually a verb (the change affected the results). "Effect" is usually a noun (the effect of the change was significant).
• Comprise vs. compose. The whole comprises the parts. The parts compose the whole. "The report comprises five sections" is correct. "The report is comprised of five sections" is technically incorrect, though widely used.
• Ensure vs. insure. "Ensure" means to make certain. "Insure" relates to insurance. In most professional writing, "ensure" is the word you want.
• That vs. which. Use "that" for restrictive clauses (essential to the sentence's meaning) and "which" for non-restrictive clauses (added information that isn't essential). "Which" is typically preceded by a comma. See our full guide to which vs. that for examples.

Grammar Mistakes Common in Academic Writing

Academic writing has its own set of common grammar and punctuation patterns that don't show up as often in everyday writing: citation punctuation, restrictive clauses around author names, multi-author possessives, semicolons in complex methodology lists, and en dashes for page ranges. If you're a researcher, graduate student, or anyone preparing academic work for submission, see our companion guide to common punctuation errors in academic writing for the academic-specific patterns.

Grammar Mistakes ESL Writers Encounter

Writers whose first language isn't English bring specific structural patterns from their first language into English writing. These aren't signs of poor ability; they're predictable consequences of how different languages handle articles, subjects, word order, and tense. A spell-checker or grammar tool can't reliably catch them because the algorithm doesn't know what language background produced the error.

Common ESL-specific patterns include:

• Article errors. Languages like Russian, Japanese, Korean, and Chinese don't use articles ("a," "an," "the") the way English does, leading to over-use, under-use, or wrong-use of articles in English writing.
• Subject omission. Languages that allow dropped subjects in everyday speech (Spanish, Italian, Japanese, Korean) lead writers to drop subjects in English where they're required.
• Passive voice overuse. Many languages prefer passive constructions in academic and formal writing, leading ESL writers to overuse passive voice in English.
• Verb tense inconsistency. Languages with different tense systems (Mandarin uses time markers rather than verb conjugation, for example) produce predictable tense inconsistencies in English writing.

For ESL writers preparing English-language manuscripts for submission, our ESL editing service connects you with native English editors who recognize and address these specific patterns rather than treating every error the same.

When to Use a Professional Editor

Reviewing your own writing for grammar mistakes is useful practice, and knowing the most common errors makes it easier to catch them before they reach your reader. For documents where accuracy and professionalism matter most, however, self-editing has its limits. Writers are often too close to their own work to catch every error, and grammar tools miss many of the mistakes covered in this article.

A professional editor reviews your writing for grammar, punctuation, clarity, and consistency at a level beyond what most writers can achieve on their own. For academic papers, dissertations, business documents, and published manuscripts, professional editing provides the level of accuracy your document requires. To understand what proofreading covers and how it differs from editing, see our article on what is proofreading.

Final Thoughts

Most common grammar mistakes come down to a small number of recurring errors: confused homophones, misplaced apostrophes, inconsistent capitalization, subject-verb and pronoun-antecedent disagreement, comma splices, dangling modifiers, and a handful of commonly confused word pairs. Knowing what to look for makes them much easier to catch and correct in your own writing.

For writing that matters, professional editing provides what self-editing and grammar tools can't. Editor World's professional proofreading services connect you with verified native English editors from the United States, the United Kingdom, and Canada. Every document is reviewed entirely by a qualified human editor with no AI at any stage. Turnaround times start at 2 hours and you choose your own editor based on credentials and verified client ratings. A certificate of editing is available as an optional add-on. Editor World is recommended by the Boston University Economics Department.

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