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, and punctuation errors. Here is 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 is an example of all three used incorrectly:


Their over they're.


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.


They're [they are] over there [in the car].


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 is an example with both words swapped:


Your sure you're order isn't ready?


Apply the same substitution test to check:


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.


  • 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 is a common example of the mistake:


Its quarter to three.


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


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.


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 cannot 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 does not 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.


Comma Splices

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.


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.


Final Thoughts

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


For more guidance on grammar and writing, visit our resources page. If your document needs professional editing, contact us or browse our editors to get started.