Modals of deduction: must, might, can't (have)
Use must, might/may/could and can't to say how sure you are about something, now or in the past, from the evidence you have.
How it works
- Present: must (I'm sure it's true), might/may/could (maybe), can't (I'm sure it's not true): She must be tired. He can't be serious.
- Past: modal + have + past participle: They must have left already. You might have dropped it in the taxi.
- The opposite of must for deduction is can't, not mustn't: It can't be him; he's in Paris.
- Couldn't have also means impossible in the past: She couldn't have seen me; I wasn't there.
- In progress now: must be + -ing: They must be sleeping; the lights are off.
- American English sometimes uses must not for negative deduction (He must not be home); can't is the safe choice.
Examples
You've been travelling all day. You must be exhausted.
The road is wet. It must have rained in the night.
He can't have finished the report already; he only started an hour ago.
I can't find my keys. I might have left them at the café.
That can't be Hoa. She's in Singapore this week.
Common mistakes
Wrong: He
mustn'tbe at home; his car isn't there.Right: He can't be at home; his car isn't there.
Wrong: She must
forgetabout the meeting yesterday.Right: She must have forgotten about the meeting yesterday.
Wrong: They might have
wenthome early.Right: They might have gone home early.
Practice
1 of 16Fill the gap.
Level: EGP Modality (deduction): must and can't for present certainty are B1; must have, might have and can't have + past participle for past deduction are controlled at B2. Written for Gloss.