20 Common English Grammar Mistakes (and How to Fix Them)

Published: May 18, 2026 · Updated: July 27, 2026 · 18 min read

Even fluent English speakers trip over the same grammar rules every day. Whether you're writing an email, a school essay, or a social media post, common grammar mistakes can undermine your message and make you look less credible. This guide covers the 20 most frequent English grammar errors — with clear wrong/right examples for each — plus a quick reference card, tips on which errors hurt your professional image most, and practical self-editing techniques.

1. Subject-Verb Agreement

The subject and verb must agree in number. Intervening prepositional phrases often disguise the true subject, making this one of the most persistent common grammar mistakes.

Wrong: The list of items are on the desk.
Right: The list of items is on the desk.

The true subject is "list" (singular), not "items." When in doubt, strip the sentence to its bare subject and verb.

2. Your vs You're

Your is possessive. You're is a contraction of "you are."

Wrong: Your going to love this.
Right: You're going to love this.

Quick test: mentally replace it with "you are." If it fits, use "you're."

3. Their vs There vs They're

Their = possessive (belongs to them). There = location. They're = contraction of "they are." Spellcheckers miss this entirely because all three are real words.

Wrong: Their going to the store over there.
Right: They're going to the store over there.

The mental check for this trio: "they're" expands to "they are"; "their" pairs with a noun (their car, their idea); "there" points to a place or introduces a clause (there is, over there). If you can replace the word with "our" and the sentence still works grammatically, you want "their."

4. Its vs It's

Its is possessive. It's is a contraction of "it is." This one tricks people because apostrophe-s usually shows possession — but not with "it."

Wrong: The cat licked it's paw.
Right: The cat licked its paw.
Right: It's raining outside.

Think of "its" as belonging to the same family as "his" and "hers" — none of them use an apostrophe for possession. When you see an apostrophe in "it's," read it aloud as "it is" to test whether the sentence still works.

5. Comma Splices

A comma splice happens when two independent clauses are joined with only a comma. Each clause could stand as its own complete sentence, so a comma alone isn't strong enough to connect them.

Wrong: I went to the store, I bought milk.
Right: I went to the store, and I bought milk.
Right: I went to the store. I bought milk.

Fix it with a coordinating conjunction (and, but, or, so), a period, or a semicolon.

6. Dangling Modifiers

A modifier at the beginning of a sentence must describe the subject. When it doesn't, you get a dangling modifier — and often an unintentionally comical sentence.

Wrong: Walking down the street, the flowers were beautiful.
Right: Walking down the street, I saw beautiful flowers.

The fix is always the same: make sure the implied subject of the opening phrase matches the actual subject of the sentence. If the sentence says "walking down the street," the subject that follows must be the one doing the walking — not the flowers.

7. Me vs I

Use I when you're the subject, me when you're the object. The trick: remove the other person from the sentence.

Wrong: The teacher gave the book to John and I.
Right: The teacher gave the book to John and me. (gave to me — not "gave to I")

The same logic applies in reverse: "John and me went to the store" is wrong because you'd never say "Me went to the store." Strip the sentence to just yourself to hear which pronoun sounds right.

8. Fewer vs Less

Use fewer for countable nouns, less for uncountable nouns. The supermarket checkout sign "10 items or less" is technically incorrect — it should be "10 items or fewer" — which tells you how deeply this mistake is embedded in everyday English.

Wrong: There are less cars on the road today.
Right: There are fewer cars on the road today.
Right: There is less traffic today.

Measurements get fuzzy ("less than 10 miles" is accepted), but for discrete items, stick with "fewer."

9. Then vs Than

Then = time/sequence. Than = comparison.

Wrong: I'm taller then my brother.
Right: I'm taller than my brother.
Right: I ate lunch, then I went for a walk.

Remember it this way: "then" has an "e" for time; "than" has an "a" for comparison. When you see "then," the sentence should involve sequence. When you see "than," a comparison is being made.

10. Who vs Whom

Who = subject (does the action). Whom = object (receives the action).

Wrong: Who did you give the book to?
Right: Whom did you give the book to? (you gave it to him — use "whom")

He who, Him whom. If you can answer with "him," use "whom." In conversation, "whom" is fading — but in formal writing, using it correctly signals competence.

11. Affect vs Effect

Affect is usually a verb (to influence); effect is usually a noun (a result). Even professionals pause to double-check this one.

Wrong: The weather had a negative affect on crop yields.
Right: The weather had a negative effect on crop yields.
Right: The weather negatively affected crop yields.

Rare exception: "effect" can be a verb meaning "to bring about" ("The CEO effected sweeping changes"). For everyday writing, affect = verb, effect = noun is right 95% of the time. Mnemonic: Affect is an Action; Effect is an End result.

12. Could Of vs Could Have

"Could've" is a contraction of "could have," not "could of." This error comes from writing what you hear — "'ve" and "of" are homophones in rapid speech.

Wrong: I could of gone to the party.
Right: I could have gone to the party.
Wrong: You should of told me earlier.
Right: You should have told me earlier.

"Have" is the auxiliary verb; "of" is a preposition that has no business in a verb phrase. If you catch yourself typing "could of," swap it for "could have."

13. Lie vs Lay

Lie = recline or rest (no direct object). Lay = put or place something (takes a direct object). The past tense of "lie" is "lay," which collides with the present tense of "lay."

Wrong: I'm going to lay down for a nap.
Right: I'm going to lie down for a nap.
Right: Please lay the book on the table.

Conjugations: Lie (recline) — lie, lay, lain, lying. Lay (place) — lay, laid, laid, laying. People lie down by themselves; you lay something down.

14. Between You and I (Hypercorrection)

Hypercorrection happens when someone tries so hard to be grammatically correct that they make an error. "Between you and I" is the classic example — people overcorrect from "John and me went" and end up using "I" everywhere, even after prepositions.

Wrong: This is just between you and I.
Right: This is just between you and me.

After any preposition (between, to, for, with, from, at), use object pronouns: me, him, her, us, them. The phrase "between you and I" sounds formal and correct to many ears, which is why it persists — but it is grammatically wrong. The test: you would never say "between I and you" or "for I," so don't say "between you and I."

15. Apostrophe Errors with Plurals

Apostrophes show possession or contraction — they do not make words plural. This is one of the most widespread common grammar mistakes, so frequent it has its own name: the "greengrocer's apostrophe."

Wrong: Apple's are on sale. / DVD's for rent. / Merry Christmas from the Smith's.
Right: Apples are on sale. / DVDs for rent. / Merry Christmas from the Smiths.

The only time an apostrophe appears in a plural is for single letters or digits to avoid confusion ("mind your p's and q's," "the 1990's" — though "1990s" is now preferred). For acronyms, just add a lowercase s: DVDs, CDs, FAQs. For family names, add -s or -es with no apostrophe: the Smiths, the Joneses. The Smith's house (possessive) versus the Smiths are coming (plural).

16. Pronoun-Antecedent Agreement

A pronoun must agree with its antecedent (the word it refers back to) in number, person, and gender. When the antecedent is a singular indefinite pronoun like "everyone," "each," or "nobody," traditional grammar requires a singular pronoun — though modern usage is shifting.

Wrong (traditional): Every student must bring their own laptop to the exam.
Right (traditional): Every student must bring his or her own laptop to the exam.
Right (modern, widely accepted): Every student must bring their own laptop to the exam.

Clear error: Each of the boys brought their lunch. → Each of the boys brought his lunch.

The singular "they" is now accepted by most style guides (APA, MLA, Chicago). In everyday writing, it avoids awkwardness. In formal academic writing, "his or her" is still safer. What matters most is consistency within the same passage.

17. Incomplete Comparisons

When you make a comparison, both sides of it must be explicitly stated. Vague comparatives like "better," "faster," or "more reliable" need a "than" clause to complete the thought — otherwise the reader is left asking "better than what?"

Wrong: Our product is faster and more reliable.
Right: Our product is faster and more reliable than the leading competitor.
Wrong: She runs faster than anyone.
Right: She runs faster than anyone else on the team.

Note a subtle variant: "She runs faster than anyone" technically includes the subject in the comparison group, creating a logical impossibility. Adding "else" fixes this.

18. Faulty Parallel Structure

When listing items or comparing ideas, all elements must follow the same grammatical form. Mixing gerunds with infinitives, or nouns with clauses, creates a jarring, unbalanced sentence that feels wrong even to readers who can't name the rule.

Wrong: She enjoys swimming, hiking, and to bike.
Right: She enjoys swimming, hiking, and biking.
Wrong: The job requires attention to detail, being organized, and that you show up on time.
Right: The job requires attention to detail, organizational skills, and punctuality.

Parallelism errors also show in correlative conjunctions (not only/but also, either/or). Whatever form follows the first half must follow the second: "She is not only intelligent but also hardworking."

19. Double Negatives

In standard English, two negative words cancel each other out and create a positive meaning. While double negatives are perfectly grammatical in some dialects and languages, in formal written English they are considered an error.

Wrong: I don't have none.
Right: I don't have any. / I have none.
Wrong: She didn't barely finish the test.
Right: She barely finished the test.
Wrong: He hasn't never been to New York.
Right: He has never been to New York.

The fix: replace the second negative with a positive term. "Any" instead of "none." "Ever" instead of "never." In creative writing, double negatives can convey character voice; in professional writing, avoid them.

20. Accept vs Except

Accept means to receive, agree to, or believe something. Except means excluding or other than. Despite looking similar, these two words have opposite functions in a sentence.

Wrong: I cannot except this award.
Right: I cannot accept this award.
Wrong: Everyone accept John came to the meeting.
Right: Everyone except John came to the meeting.
Right: We accept all major credit cards except Discover.

Think of "ex" in "except" as meaning "exclude" — both start with "ex" and mean leaving out. "Accept" starts with "ac" like "acquire" — both involve taking in. When proofreading, pause on every "accept" and "except" to verify.

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Why Native Speakers Make These Mistakes

If these rules are taught in school, why do native speakers still make these common grammar mistakes well into adulthood? The answer lies in how we acquire language versus how we learn formal rules.

Spoken language leads, written rules follow. Children learn by listening and imitating — not by memorizing textbooks. By the time formal grammar instruction begins, deeply ingrained speech patterns are already set. The ear overrides the rulebook in the moment of writing.

Homophones are invisible to the ear. Your/you're, their/there/they're, its/it's — these pairs sound identical in speech. Since we subvocalize when we write, the distinction doesn't register unless we pause deliberately. It's a writing-specific problem that requires writing-specific vigilance.

Hypercorrection creates new errors. People overapply rules: knowing "John and me went" is wrong, they conclude "and I" is always correct — even after prepositions where "me" is required. The speaker cares about grammar but hasn't fully internalized the rule's scope.

Frequency normalizes errors. "10 items or less" appears on signs everywhere; "who did you give it to" dominates speech over "whom." Ubiquitous exposure to common grammar mistakes erodes our internal correctness meter, even if we once knew the rule.

Mistakes by Category

Grouping the 20 common grammar mistakes by the type of rule they break helps you identify patterns in your own writing.

Punctuation Errors

• Comma Splices (#5) — Two complete sentences joined with only a comma.
• Apostrophe Errors with Plurals (#15) — Using an apostrophe to form a plural.

Word Confusion

The largest category — words that sound similar but have different meanings:

• Your vs You're (#2)
• Their vs There vs They're (#3)
• Its vs It's (#4)
• Then vs Than (#9)
• Affect vs Effect (#11)
• Could Of vs Could Have (#12)
• Fewer vs Less (#8)
• Lie vs Lay (#13)
• Accept vs Except (#20)

Subject-Verb & Pronoun Agreement

• Subject-Verb Agreement (#1) — Verb must match the subject in number.
• Pronoun-Antecedent Agreement (#16) — Pronouns must match their antecedents.

Modifier & Structure Errors

• Dangling Modifiers (#6) — Modifier doesn't describe the intended subject.
• Incomplete Comparisons (#17) — Comparison lacks a clear second term.
• Faulty Parallel Structure (#18) — Listed items don't share the same form.
• Double Negatives (#19) — Two negatives cancel each other.

Pronoun Errors

• Me vs I (#7) — Confusing subject and object pronouns.
• Who vs Whom (#10) — Same subject/object confusion.
• Between You and I (#14) — Hypercorrection using "I" after a preposition.

Quick Reference Card

Bookmark this table for a one-glance reminder of the most frequent errors and their fixes:

Keep this table handy — fixing these common grammar mistakes consistently will sharpen your writing faster than memorizing dozens of less frequent rules.

Grammar Mistakes That Make You Look Unprofessional

In professional contexts — job applications, client emails, reports — certain errors carry a disproportionate credibility cost. Readers forgive obscure grammar points but not mistakes any competent writer should catch.

Your vs You're in formal correspondence. An email that begins "Your invited to the meeting" signals carelessness. In a Grammarly survey of hiring managers, spelling and grammar errors in cover letters were the number one reason for immediate rejection.

Apostrophe errors with plurals. "The report's are due Friday" on a business document suggests a fundamental gap in writing skills, especially damaging in client-facing materials where your writing represents company standards.

Could of / should of. "I could of sent the file yesterday" in an email looks worse than a typo — it suggests the writer transcribes speech sounds rather than understanding word meaning.

Affect vs Effect in business writing. In marketing, medicine, law, and finance, these words appear frequently. Getting them wrong — "The policy change will effect our bottom line" — confuses meaning and undermines authority in your own field.

Subject-verb agreement in formal writing. "The data shows that our strategy are working" makes readers question whether you reviewed your work. Agreement errors undermine the clarity professional writing demands.

How to Catch Your Own Mistakes

Knowing the rules is half the battle. The other half is training yourself to spot these common grammar mistakes in your own writing before you hit send. Here are seven practical self-editing techniques:

1. Read aloud. Your brain autocorrects mistakes when reading silently. Reading aloud forces you to process every word and hear awkward phrasing. If a sentence feels awkward to say, it reads awkwardly too.

2. Read backwards. Start at the last sentence and work to the first. This breaks narrative flow and makes each sentence stand alone. Errors hidden by context suddenly become visible — especially useful for word confusion like affect/effect.

3. Expand contractions. For homophone errors, mentally expand each contraction. Replace "you're" with "you are," "it's" with "it is." If the sentence breaks, you have the wrong word.

4. Change the medium. Print your document or change the font. Visual novelty tricks your brain into seeing the text as new material. Errors become visible simply because they look different.

5. Let it rest. Even 10 minutes away resets your mental model. For important pieces, review the next morning — it catches errors same-day proofreading misses.

6. Use an AI grammar checker. Automated tools catch a different class of errors than human review. Our free grammar checker highlights mistakes you may have read past three times. Run it as a final pass after your own self-editing.

7. Track your patterns. Most writers repeat the same two or three errors. Identify yours and check for them specifically. A targeted check beats a vague "look for mistakes" pass every time.

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