The Top 20 Common Errors Checklist
You've got a red mark, a green squiggle, or just that quiet feeling a sentence is off — and you don't want a lecture, you want the fix. So here it is. Scan the Symptom column until something matches, run the one-line fix, and if it still nags, take the link home to the full article for the why. Bookmark this. It's the clinic door, not the seminar.
| Symptom (the mark-up or the feeling) | One-line fix | Where to go |
|---|---|---|
| 1. its / it's — apostrophe flagged, or you froze mid-word | Say it is / it has in your head: if that works, write it's; if not, its. | Its vs it's · Pillar 2 |
| 2. their / there / they're — squiggle under one, or a hesitation | Ownership → their; place or there is → there; they are → they're. | Their, there, they're · Pillar 8 |
| 3. Subject–verb agreement — "S-V AGR" (the list of names are…) | Strip everything between subject and verb; match the head noun. The list… is. | Subject–verb agreement · Pillar 5 |
| 4. Collective nouns — the team is / are anxiety | Unit → singular; the people in it → plural. Just be consistent. UK/US: UK takes either (the team are); US usually singular (the team is) — house style may override. | Collective-noun agreement · Pillar 5 |
| 5. Sentence fragment — "frag", or it reads chopped-off | Check for a subject and a finite verb; if one's missing, add it or join it to the sentence beside it. | Fragments · Pillar 3 |
| 6. Run-on — two full thoughts jammed together, no join | Split into two sentences, or join properly (full stop, semicolon, or a comma plus and / but / so). | Run-ons · Pillar 3 |
| 7. Comma splice — "comma splice"; a lone comma between two sentences | Swap the comma for a full stop or semicolon, or add a joining word. A comma can't carry two sentences alone. | Comma splices & semicolons · Pillar 6 |
| 8. Dangling modifier — opening phrase attached to the wrong thing (Walking home, the rain started.) | The first noun after the comma must be the one doing the action — name it, or rebuild. | Dangling & misplaced modifiers · Pillar 3 |
| 9. a / an / the — article missing or wrong | a before a consonant sound, an before a vowel sound; use the when the reader can name which one. | Articles: a, an, the · Pillar 2 |
| 10. Preposition choice — different to / from, at / on the weekend | Pick the settled pairing for your reader. UK/US: UK at the weekend, different to/from; US on the weekend, different from/than — region, not wrongness. | Prepositions · Pillar 2 |
| 11. affect / effect — spellcheck's no help and you always pause | Usually affect = verb (to influence), effect = noun (the result). If the fits in front, you want effect. | Affect vs effect · Pillar 8 |
| 12. principle / principal — which "-ple/-pal"? | Principle = a rule or belief; principal = the main one, or the head of a school. The principal is your pal. | Principle vs principal · Pillar 8 |
| 13. fewer / less — "countable?" beside a comparison | Count them one by one (people, errors) → fewer; a bulk amount (water, time) → less. | Fewer vs less · Pillar 2 |
| 14. Pronoun case — me and him went, between you and I | Strip the other person: me went? → I went; between… I? → between me. Keep whichever still sounds right alone. | Pronoun case (I/me) · Pillar 2 |
| 15. whom overcorrection — whom dropped in to sound proper | Test with he/him: he → who, him → whom. If you'd say he did it, it's who did it. | Who vs whom · Pillar 2 |
| 16. Tense shifting — "tense jump"; drifts past ↔ present for no reason | Pick one main tense for the stretch and hold it; only shift when the time genuinely changes. | Tense consistency · Pillar 4 |
| 17. Vague "it / this" — "unclear reference" after This shows… | Put a noun right after it: This delay…, This result… — never make the reader guess. | Vague pronouns & antecedents · Pillar 5 |
| 18. Double negatives — "DN" (I don't need no help) | Keep one negative per clause: I don't need any help or I need no help, not both. | Negation & double negatives · Pillar 2 |
| 19. Faulty parallelism — a list of mismatched forms (reading, to swim, and tennis) | Dress every item the same: all -ing, all to- verbs, or all nouns. Reading, swimming, and tennis. | Parallel structure · Pillar 3 |
| 20. Unfinished comparison — ours is faster (than what?) | Finish the thought: name the other side. Ours is faster than theirs. | Comparisons · Pillar 3 |
Two rows above carry a real UK/US split worth flagging — collective nouns and a handful of prepositions. Everything else here is shared ground in standard written English; the spelling toggles you'll meet inside the linked articles (colour [US: color]) aren't errors at all.
Keep this open beside the draft. The good news is that half the red pen vanishes the moment you've run the one-liner — and the full clinic is one click away when you want it.
Full set: all 17 Pillar 10 diagnostic articles · Grammar Clinic Hub