Grammar / usage: Modal verbs and their shades of meaning
Subtopic: Section D · Grammar
How to structure your answer
Rule: modal verbs (can, could, may, might, shall, should, will, would, must, ought to, need, dare) take the bare infinitive and do not change form for person or number. They express ability, permission, obligation, advice, possibility, certainty, habit and prediction — shades that English's main verbs cannot mark on their own. Past-tense modals (could, might, should have) signal not only past time but also weaker possibility or tentativeness.
Approach: identify the shade of meaning the writer needs — ability vs permission, advice vs obligation, possibility vs certainty — and choose the modal that marks that shade. Watch the perfect modals ('must have arrived' for past deduction, 'should have arrived' for past expectation unmet).
What an examiner expects: the rule stated cleanly; corrected sentences with the modal change underlined; a one-line justification.
Common pitfalls: (1) using 'can' for permission in formal writing ('Can I leave?' instead of 'May I leave?'); (2) using 'must' for personal advice instead of 'should'; (3) collapsing 'must have', 'might have' and 'should have' into one form.
Detailed model answer
124 words · target 100 words · 8 min
Rule: Modal verbs (can, could, may, might, must, shall, should, will, would) carry shades of possibility, permission, obligation and advice. Choose by the strength and the certainty intended.
Worked examples: 'You must wear a helmet' (legal compulsion). 'You should wear a helmet' (advice). 'It may rain' (open possibility). 'It might rain' (slightly weaker possibility). 'He can swim' (ability). 'He could swim as a child' (past ability).
Quick check:
- You (must/should) submit the form today — it is the deadline. must
- She (can/could) play the violin when she was ten. could
- (May/Can) I borrow your pen, please? May (formal request)
- He (must/might) be tired — he has worked all day. must (deduction)
- Students (shall/will) sit only in the assigned hall. shall (formal regulation)
What an examiner expects to see
- Ability: can (present), could (past or polite request).
- Permission: may (formal), can (informal); 'might' is the polite version of 'may'.
- Obligation: must (strong, internal), have to (external), should/ought to (advice).
- Possibility / certainty: may, might, could (less certain), must (deduction), will (prediction).
- Past modals carry past time AND weaker tone — could, might, should.
- Perfect modals — past time only: 'must have arrived' (deduction), 'should have arrived' (expectation unmet), 'might have arrived' (uncertainty).
- Modals take the bare infinitive — no 'to', no -s in third person.
- Two modals cannot be combined ('will can' is wrong; use 'will be able to').
- Negative forms: cannot / mustn't / shouldn't / mightn't — note 'mustn't' (prohibition) differs from 'needn't' (absence of obligation).
- Practice sentences for the examinee to correct.
Concrete cases, schemes and judgments
- Wrong: 'He can to swim.' Right: 'He can swim.' (modals take bare infinitive — no 'to'.)
- Wrong: 'You must consult a doctor for general advice.' Right: 'You should consult a doctor.' (advice — should, not must.)
- Wrong: 'He must have came earlier.' Right: 'He must have come earlier.' (perfect modal — past participle.)
- Wrong: 'She needn't to apologise.' Right: 'She need not apologise.' OR 'She doesn't need to apologise.' ('Need' as modal takes bare infinitive.)