Grammar rules, explained
These six sentence-level rules account for most of the red ink on essays and reports. Each one comes with a before-and-after you can flip through, so you see the mistake and the fix in the same sentence.
- Passive voiceA sentence is passive when its subject receives the action instead of doing it.
- Subject–verb agreementThe verb must match its subject in number: The dog barks, The dogs bark.
- Run-on sentencesA run-on (or fused) sentence jams two independent clauses together with no proper join: I love hiking it clears my head.
- Comma splicesA comma splice is two complete sentences joined by nothing but a comma: The store was closed, we went home.
- Dangling modifiersAn opening phrase describes whoever comes right after the comma.
- Parallel structureParallel structure (parallelism) means items that do the same job in a sentence take the same grammatical form.
Word-level mistakes
The other big group of common grammar mistakes is word choice: pairs like its and it's, who and whom or fewer and less. Those live in the commonly confused words list.
Rules you can stop worrying about
What are the most common grammar mistakes?
In student and professional writing: confusing sound-alike words (its/it's, their/there/they're, affect/effect), comma splices and run-on sentences, subject–verb agreement errors, dangling modifiers, and lists that are not parallel.
Is it OK to start a sentence with "and" or "but"?
Yes. Both Chicago and Merriam-Webster note that this has been done by respected writers for centuries. It is a style choice, not a grammar error.
Can I end a sentence with a preposition?
Yes. "What are you looking at?" is standard English. The "rule" against it came from applying Latin grammar to English and is rejected by modern usage guides.