Writing and Language
25 Common Grammar Mistakes (and How to Fix Them)
By AIDetector365 Editorial Team · 14 min read · Published · Updated
A handful of grammar mistakes account for most of the errors in everyday writing. Native speakers make them in emails, students make them in essays, and autocorrect often introduces them. The good news: once you know the pattern, each one is easy to fix.
Below are 25 common grammar mistakes in American English with examples and fixes. Many of them are caught automatically by our free grammar checker, which explains every correction and runs entirely in your browser.
Key takeaways
- Most mistakes fall into five groups: confused words, agreement, apostrophes, punctuation and capitalization.
- Homophones (their/there/they're, its/it's, your/you're) are the most frequent errors.
- Read your text aloud: many agreement and missing-word errors become obvious.
- A grammar checker catches common patterns but not every error; proofread too.
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Confused words
| # | Mistake | Correct | Rule |
|---|---|---|---|
| 1 | Their going to be late. | They're going to be late. | They're = they are. |
| 2 | Put it over their. | Put it over there. | There = a place or existence. |
| 3 | There prices are high. | Their prices are high. | Their = belonging to them. |
| 4 | Its a nice day. | It's a nice day. | It's = it is / it has. |
| 5 | The dog wagged it's tail. | The dog wagged its tail. | Its = belonging to it (no apostrophe). |
| 6 | Your welcome. | You're welcome. | You're = you are. |
| 7 | She is taller then me. | She is taller than me. | Than compares; then is about time. |
| 8 | The weather will effect the game. | The weather will affect the game. | Affect is usually the verb; effect the noun. |
| 9 | I could care less. | I couldn't care less. | The idiom means you care as little as possible. |
| 10 | Loose the weight. | Lose the weight. | Loose = not tight; lose = misplace or shed. |
For forty more pairs, see Commonly confused words in English.
Verb and agreement mistakes
11. Could of, should of, would of
“I should of called.” → “I should have called.” The spoken contraction “should've” sounds like “should of.”
12. Subject-verb agreement with he/she/it
“She don't like it.” → “She doesn't like it.” “He have a car.” → “He has a car.”
13. Plural subjects with singular verbs
“The prices is high.” → “The prices are high.” “They was late.” → “They were late.”
14. Agreement across a phrase
“The box of chocolates are on the table.” → “The box of chocolates is on the table.” The verb agrees with “box,” not “chocolates.”
15. “Me and him went”
“Me and him went to the store.” → “He and I went to the store.” Use subject pronouns for subjects. Test by removing the other person: “I went,” not “me went.”
16. Shifting tenses
“She walks in and said hello.” → “She walked in and said hello.” Keep tense consistent unless time actually changes.
Articles and word-level mistakes
17. A vs. an
Use “an” before a vowel sound, not a vowel letter: “an hour,” “an MBA,” but “a university,” “a one-time fee.”
18. Repeated words
“We went to the the store.” Easy to miss when proofreading; easy for a checker to catch.
19. Alot
“Alot” is not a word. Write “a lot,” or use “many” or “much” in formal writing.
20. Common misspellings
“Definately” → “definitely,” “recieve” → “receive,” “seperate” → “separate,” “occured” → “occurred,” “untill” → “until.”
Punctuation and capitalization mistakes
21. Lowercase “i”
“i think so” → “I think so.” The pronoun I is always capitalized.
22. Missing capital at the start of a sentence
“it rained. we stayed in.” → “It rained. We stayed in.”
23. Space before punctuation
“Really ?” → “Really?” In English, no space goes before commas, periods, question marks or colons.
24. Missing space after punctuation
“Yes,we can.” → “Yes, we can.”
25. Comma splices
“It was late, we went home.” → “It was late, so we went home.” or “It was late. We went home.” Two complete sentences need more than a comma.
What a grammar checker catches — and what it misses
The AIDetector365 grammar checker is rule-based and follows American English. It catches many of the mistakes above when they follow common patterns — “there prices,” “its a,” “your welcome,” “then” in comparisons, “could of,” “she don't,” “they was,” “me and him” as a subject, a/an, repeated words, “alot,” common misspellings, lowercase “i” and spacing around punctuation — and it explains each fix. Like any rule-based checker, it does not catch everything: “their going,” “it's tail,” agreement across long phrases, comma splices, tense shifts and words that are spelled correctly but wrong in context need a human read.
- Paste your text into the grammar checker and click Check Grammar.
- Read each suggestion and its explanation; accept the ones that are right for your sentence.
- Copy the corrected version.
- Read the final text aloud to catch what no tool can.
Proofreading checklist
- Search your text for “their,” “there,” “its,” “your” and “then” and check each one.
- Read each sentence's subject and verb together, skipping the words in between.
- Check every apostrophe: is it a contraction or possession?
- Read the text backwards sentence by sentence to spot repeated words.
- Run a final pass with a grammar checker, then a final read aloud.
For a broader guide to proofreading tools and techniques, read our grammar and spell check guide.
Frequently asked questions
What is the most common grammar mistake?
Confusing homophones such as their/there/they're, its/it's and your/you're is among the most frequent errors in everyday writing.
Is “me and him” ever correct?
Yes, as an object: “She called him and me.” As a subject, use “He and I went.” Putting yourself last is the polite convention.
Is it “should of” or “should have”?
Always “should have” (or “should've”). “Should of” comes from how the contraction sounds.
Does the grammar checker support British English?
Suggestions follow American English. British spellings may be shown as differences, not errors.
Is my text stored when I check grammar?
No. The checker runs in your browser and your text is never uploaded.
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