15 Common Grammar Mistakes and How to Fix Them

The most common grammar mistakes in English are mixing up words that sound alike (their, there, they're; its, it's; your, you're; then, than), writing "could of" instead of "could have", "alot" instead of "a lot", and leaving a lowercase "i". Each has a simple test you can learn in seconds. A free Grammar & Writing Checker flags these automatically as you type.

1. Their, there and they're

• their = belonging to them: "their house"
• there = a place, or "there is": "over there", "there are three"
• they're = they are: "they're late"

Test: if you can say "they are", write they're.

2. Its and it's

• it's = it is or it has: "it's raining", "it's been a week"
• its = belonging to it: "the company changed its name"

Test: try "it is". If it fits, use the apostrophe. Possessive its never has one.

3. Your and you're

• you're = you are: "you're right"
• your = belonging to you: "your phone"

4. Then and than

• than compares: "better than before"
• then is about time: "first this, then that"

5. Could of, should of, would of

Always could have, should have, would have (or could've). "Of" sneaks in because "could've" sounds like it.

6. Alot

Write a lot. (Allot means to share out: "allot 10 minutes to each speaker".)

7. Lowercase "i"

The pronoun I is always a capital letter, even in the middle of a sentence.

8. Affect and effect

• affect is usually the verb: "the rain affected the match"
• effect is usually the noun: "the rain had an effect"

9. Lose and loose

• lose = to fail to keep: "don't lose your keys"
• loose = not tight: "a loose screw"

10. To, too and two

• to: direction or before a verb: "go to school", "to win"
• too: also, or excessive: "me too", "too hot"
• two: the number 2

11. A or an

Use an before a vowel sound, not just a vowel letter: "an hour", "an MBA", but "a university", "a one-time fee".

12. Repeated words

"The the" and "very very" slip in when you edit. They're hard to see when you reread your own writing, which is why checkers are good at catching them.

13. Missing capital at the start of a sentence

Every sentence starts with a capital letter. This often goes wrong after editing or pasting.

14. Spaces around punctuation

No space before a comma, full stop, question mark or exclamation mark; one space after. One exclamation mark is enough!

15. Wordy phrases

Not a grammar error, but it weakens your writing:

Instead of	Write
utilize	use
in order to	to
at this point in time	now
due to the fact that	because
very unique	unique

How does the Grammar & Writing Checker work?

1. Open the Grammar & Writing Checker.
2. Type or paste your text, or press Try a sample to see it in action.
3. Issues are highlighted in the text and listed with an explanation, grouped as spelling, grammar, style, clarity and punctuation. Filter by category.
4. Click a suggestion to apply it, Ignore to skip it, or Add to my dictionary for names and words you use.
5. Check the writing score and readability panel, then Copy text.

It uses a US-English spelling dictionary plus grammar and style rules, and runs entirely in your browser in a background worker, so even long documents stay private and typing stays smooth. It's not AI: it catches common, rule-based mistakes, but it can't judge meaning, tone or facts. Always reread important writing.

Try it on this sentence: "Their going too the park alot. i could of went." The checker suggests They're, to, a lot, I and could have.

Quick checklist before you send

• Read it aloud. Your ear catches what your eye skips.
• Check the homophones above.
• Run a checker for spelling and repeated words.
• Shorten long sentences; see how to improve your readability score.

Check your writing

Paste your text into the free Grammar & Writing Checker. Then measure it with the Readability Checker, count it with the Word Counter, and fix headings with the Case Converter; see also title case vs sentence case.