Modal verbs · Upper-intermediate (B2)
Modals of deduction: must, might, can’t
Say how sure you are: It must be true. She might be late. He can’t have seen us.
The rules
Present: must = I’m sure it’s true (She must be tired — she’s been working all day); might / may / could = it’s possible (He might be in a meeting); can’t = I’m sure it’s not true (That can’t be Tom — he’s in Brazil).
Past: modal + have + past participle: They must have left already. She might have forgotten. He can’t have seen us.
Note: the opposite of must (deduction) is can’t, not mustn’t.
Examples
- The lights are off. They must be out.
- I’m not sure where she is. She could be at the gym.
- You can’t be hungry — you’ve just eaten!
- The road is wet. It must have rained.
Common mistakes
✗ He mustn’t be at home; his car isn’t there.
✓ He can’t be at home; his car isn’t there.
Negative deduction = can’t.
✗ She must forgot.
✓ She must have forgotten.
Past deduction = modal + have + participle.
Practice
She’s not answering. She ___ be asleep.
Quiz
1.He’s won the lottery! He ___ be very happy.
2.I can’t find my keys. I ___ have left them at work.
3.“Can’t have + participle” is for a past deduction that something was impossible.
4.Put the words in the right order.