The 12 Grammar Mistakes That Cost Students the Most Marks

Ask any marker: the same dozen grammar errors appear in essay after essay. None of them reflects intelligence — they're habits, and habits can be fixed. Here are the most costly ones.

'Should of' instead of 'should have' tops the list because it's invisible to the writer: the contraction 'should've' sounds exactly like 'should of', so the ear approves what the eye should reject. The same trap catches 'could of' and 'would of'. Next come the homophone swaps — their/there/they're, its/it's, your/you're — which spellcheckers can't catch because each word is correctly spelled, just wrongly chosen.

Comma splices join two complete sentences with only a comma: 'The experiment failed, we started over.' Fix with a full stop, a semicolon, or a conjunction. Run-on sentences are the same disease in a worse stage — three or more clauses chained without adequate punctuation, exhausting the reader.

Apostrophe errors cluster in two forms: missing from contractions ('cant', 'dont') and wrongly inserted into plurals ('the student's were late'). Subject-verb disagreement sneaks in when a phrase separates subject from verb: 'The quality of the arguments were poor' — the subject is 'quality', so the verb must be 'was'.

Rounding out the list: dangling modifiers ('Walking to class, the rain started'), inconsistent tense within a paragraph, unclear pronoun reference ('Sarah told Emma that she had won' — who won?), 'alot' for 'a lot', and misplaced 'only' ('I only ate breakfast' vs 'I ate only breakfast').

The practical fix: proofread once specifically for these patterns — not for meaning, just mechanics — and run your final draft through a free grammar checker that explains each correction. The explanation matters more than the fix; it's how the habit changes.