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Les verbes modaux expriment des nuances de sens : possibilité, obligation, permission, déduction. Can/Could : capacité et permission. May/Might : possibilité. Must/Have to : obligation. Must not : interdiction. Don't have to : absence d'obligation. Should/Ought to : conseil. Shall : proposition ou promesse formelle. Would : conditionnel et politesse. Pour la déduction : 'must' (certitude positive), 'can't/couldn't' (certitude négative), 'might/could/may' (possibilité). Exemple : 'He must be tired' (je suis sûr qu'il est fatigué). 'She can't be at home' (je suis sûr qu'elle n'est pas chez elle). 'They might be late' (peut-être qu'ils sont en retard). Les modaux + have + participe passé expriment ces nuances dans le passé : 'He must have forgotten' (il a sûrement oublié).
Exercice 1 : Choisissez le modal correct.
1. You ___ (must/should) see a doctor — you look very ill.
2. She ___ (can/might) speak three languages.
3. You ___ (mustn't/don't have to) smoke here — it's forbidden.
4. He ___ (could/should) have called me — I was worried.
5. They ___ (may/must) be students — they have backpacks.
Exercice 2 : Réécrivez avec un modal.
1. It is possible that she forgot. → She ___
2. I am certain he is lying. → He ___
3. It is not necessary to wear a tie. → You ___
What is the difference between 'must' and 'have to'?
Correction
Both express obligation, but 'must' is typically internal (personal decision or strong conviction): 'I must study harder.' 'Have to' usually expresses external obligation (rules, authority): 'I have to wear a uniform at school.' In the negative, 'mustn't' = prohibition, while 'don't have to' = no obligation (you are free not to do it).
How do you use 'should have + past participle'?
Correction
'Should have + past participle' expresses regret or criticism about a past action. Example: 'You should have studied more' (but you didn't — it was a mistake). It implies the action did not happen but should have.
How do you express deduction in English using modals?
Correction
Use 'must' for near-certain positive deduction ('He must be the teacher — everyone respects him'), 'can't/couldn't' for near-certain negative deduction ('She can't be tired — she just woke up'), and 'might/could/may' for uncertain possibility ('They might be in the library').