The 30 most common grammar mistakes (and how to fix each)

Quick answer. Most manuscript errors fall into a handful of habits, not hundreds of separate rules: the wrong homophone (their/there/they're), a mismatched subject and verb, a misplaced comma, a dangling modifier, and tense that drifts mid-paragraph. Fix those five families and you have removed much of what a proofreader would flag. The list below is the 30 that come up most often, grouped so you can check your own draft in an afternoon.

Key takeaways

• Most mistakes are habits, not knowledge gaps — you know the rule; you slip because you are reading meaning, not words.
• Homophones and comma placement top the list. They are also the fastest to fix with a targeted search.
• Read backwards, sentence by sentence, to catch what your brain auto-corrects on a normal read.
• A grammar check is not a proofread. Tools miss agreement across a clause and misread intent; a human pass catches what they can't.

The 30, in five families

A. Homophones and confusables (1–8)

1. their / there / they're — possession / place / they are. Test: swap in they are; if it fits, it's they're.
2. your / you're — possession / you are. Same swap test.
3. its / it's — possession / it is or it has. Its takes no apostrophe when it means "belonging to it".
4. to / too / two — direction or infinitive / also or excessive / the number. Too has the extra o meaning "extra".
5. then / than — time or sequence / comparison. Then answers "when"; than follows a comparison.
6. affect / effect — usually a verb (to influence) / usually a noun (a result). Rare uses flip, but the common case is this.
7. who's / whose — who is / possession.
8. compliment / complement — praise / completes or matches. Complement = complete.

B. Subject–verb agreement (9–14)

1. A singular subject with a plural verb — The list of names are long → is. The subject is list, not names.
2. Compound subjects — The author and the editor is → are. Two subjects need a plural verb.
3. Collective nouns — The team are vs The team is. Pick one convention and hold it; US usage is usually singular.
4. Tricky singulars ending in -s — The news are → is; mathematics, politics and economics are singular.
5. "One of the…" — One of the books are → is. The subject is one.
6. Indefinite pronouns — Everybody, each, everyone, none (usually) take a singular verb: Everyone has a draft.

C. Punctuation (15–21)

1. Comma splice — two full sentences joined by only a comma. Use a full stop, a semicolon, or a conjunction.
2. Missing serial comma where it prevents ambiguity — editors, authors and agents reads as two groups; add the comma if a list item can pair with its neighbour.
3. Apostrophe for plural — three book's → books. No apostrophe in a plain plural.
4. Apostrophe placement in possessives — the author's books (one) vs the authors' books (many). Put it after the s for plurals.
5. Quotation marks with other punctuation — US style puts the comma and full stop inside the closing quote: "Done," she said.
6. Hyphen vs dash — hyphen joins a compound (well-known); em dash marks a break in thought. They are different marks.
7. Question mark with a reported question — He asked whether she was finished? → full stop. A reported question is not a direct one.

D. Sentence structure (22–27)

1. Dangling modifier — Walking to the café, the manuscript was lost. The manuscript wasn't walking. Name the actor.
2. Misplaced modifier — She only edited two chapters vs She edited only two chapters. Where the modifier sits changes the meaning.
3. Run-on sentence — two or more independent clauses with no punctuation. Split, or join with a conjunction.
4. Faulty parallelism — editing, proofreading, and to format → all items must share the same form: editing, proofreading, and formatting.
5. Passive voice used by default — not wrong, but if every sentence is passive the prose goes flat. Keep it for when the actor is unknown or unimportant.
6. Sentence fragments presented as sentences — fine in fiction for effect, a problem in exposition. Make sure the fragment is a deliberate choice.

E. Consistency and word choice (28–30)

1. Tense drift — slipping from past to present mid-paragraph. Choose a dominant tense and hold it; shifts should be deliberate.
2. Pronoun with no clear antecedent — When the editor met the author, she was late. Who is she? Name them.
3. Redundancy and wordiness — free gift, advance planning, in order to, very unique. Cut the word that repeats or weakens the meaning.

A fast self-edit pass

1. Search, don't scan. Run a find for each homophone pair (its/it's, your/you're, then/than, affect/effect) and check every hit in context.
2. Read backwards, one sentence at a time. Your brain stops auto-correcting when the meaning no longer flows — slips surface.
3. Check every comma splice and every long sentence for its subject and verb. Agreement errors hide in long clauses.
4. Hunt modifiers. Every opening -ing phrase should describe the subject of the clause that follows it.
5. Pick your conventions and note them — serial comma yes/no, US or UK punctuation, one tense. A one-page style sheet stops the same slips returning.
6. Then have a human read it. A grammar tool catches the mechanical ones; it misreads intent, misses agreement across a clause, and cannot tell a deliberate fragment from a mistake.

That final human read — every chapter, both the mechanical slips and the sentences a tool waves through — is exactly what a full edit covers. If your manuscript is finished and you want that pass, our editing and proofreading service is where it happens.

Related guides

Keep going: how to write a book, the types of book editing, book editing services and proofreading services. For a dictionary definition, see grammar (Cambridge Dictionary).

Disclosure: Harmony Publishing is a self-publishing company and links to its own services where they match a step. The language guidance here is practical, not a rule for every style guide.