12 common grammar mistakes and how to fix them

The mistakes below are not signs of poor writing. They are the predictable traps of English, and even strong writers fall into them under time pressure. The good news is that each one has a quick test. Learn the test, apply it when you hit the tricky word, and the error stops slipping through. They are grouped loosely, from the apostrophe confusions to the word pairs to the structural slips.

The apostrophe pairs: its, your, their, whose

1. Its vs it's.This is the most common written error in English. "It's" is always a contraction of "it is" or "it has". "Its" is the possessive. The trick is to expand it: if "it is" fits, use the apostrophe. "It's raining" expands to "it is raining", so the apostrophe is right. "The dog wagged its tail" cannot become "it is tail", so no apostrophe. Possessive pronouns (its, his, hers, ours, theirs) never take apostrophes.

2. Your vs you're.Same logic. "You're" is "you are". "Your" shows possession. "You're late" is "you are late". "Your coat" cannot become "you are coat". If you can swap in "you are", use the apostrophe.

3. Their vs there vs they're.Three words, three jobs. "They're" is "they are". "There" is a place or the dummy subject in "there is". "Their" is possessive. "They're bringing their books over there" uses all three correctly. Expand the contraction to test it, and ask whether you mean a place (there) or ownership (their).

4. Whose vs who's."Who's" is "who is" or "who has". "Whose" is possessive. "Who's coming?" is "who is coming". "Whose coat is this?" asks about ownership, so no apostrophe. The pattern across all four is identical: the apostrophe form is a contraction, the plain form is possessive.

The verb-noun pairs: affect, effect

5. Affect vs effect. In everyday use, "affect" is the verb (to influence) and "effect" is the noun (the result). The mnemonic is RAVEN: Remember, Affect is a Verb, Effect is a Noun. "The weather affects my mood" uses the verb; "the effect was immediate" uses the noun. There are rarer uses (effect can be a verb meaning to bring about, as in "effect change", and affect can be a noun in psychology), but the verb-affect, noun-effect split covers almost every sentence you will write.

The countable pairs: fewer, less, amount, number

6. Fewer vs less. Use "fewer" for things you can count individually and "less" for things you measure as a mass. "Fewer mistakes", because you can count mistakes; "less confusion", because you cannot count confusion. The supermarket sign that reads "ten items or less" should say "ten items or fewer", because items are countable. The test: if a number can sit in front of the noun, use "fewer".

7. Amount vs number. The same split. Use "number" for countable things ("a number of errors") and "amount" for mass quantities ("an amount of water"). "A large amount of people" is wrong; people are countable, so it should be "a large number of people".

The sound-alike pairs: then, than, lose, loose

8. Then vs than. "Then" is about time or sequence ("we ate, then we left"). "Than" is for comparisons ("bigger than a bus"). They sound similar in fast speech, which is why they get swapped in writing. If you are comparing two things, it is always "than".

9. Lose vs loose."Lose" is the verb (to misplace, or to not win): "don't lose your keys". "Loose" is the adjective (not tight): "a loose screw". The extra "o" in "loose" is the giveaway; it rhymes with "goose" and means slack. "Lose" rhymes with "news".

The structural slips: who, whom, me, myself

10. Who vs whom. "Who" is the subject (the one doing the action); "whom" is the object (the one receiving it). The test: answer the question with "he" or "him". If the answer is "him", use "whom" (both end in m): "Whom did you call?" answers "I called him". If the answer is "he", use "who": "Who called?" answers "he called". In casual writing "who" is widely accepted in both roles, but in formal writing the distinction still counts.

11. Me vs I vs myself. The trap appears in pairs. "Me and Sam went" should be "Sam and I went", and "between you and I" should be "between you and me". The test is to drop the other person: you would say "I went", not "me went", and "between us", which takes the object form "me". And "myself" is not a fancy "me". Use it only to reflect back on the subject ("I taught myself") or for emphasis, never as in "please contact myself", which should be "please contact me".

The agreement slip: subject and verb

12. Subject-verb agreement with long subjects. The verb must agree with the true subject, not with the nearest noun. "The box of files are heavy" is wrong; the subject is "box" (singular), not "files", so it should be "the box of files is heavy". A long phrase between the subject and the verb is where this slips. Find the real subject, ignore the words in between, and match the verb to it. The same care applies to "each", "everyone", and "neither", which are singular: "each of the players is ready", not "are ready".

A proofreading habit that catches these

These errors survive a normal read because the eye glides over familiar words. Two habits flush them out. First, read the draft aloud, slowly; the ear catches a "then" that should be "than" when the eye does not. Second, when you reach one of these trigger words, stop and run its test rather than trusting your first instinct. "It's" or "its"? Expand to "it is" and check. "Who" or "whom"? Answer with "he" or "him". The test takes two seconds and turns a guess into a decision.

What to do next

Run your draft through the Phrasit grammar checker, which flags many of these confusions automatically so you can apply the right fix. Use the reading level analyzer to spot the long, tangled sentences where subject-verb agreement tends to go wrong, and the find and replace toolto hunt down a specific habitual slip, like every stray "your" you tend to type for "you're".