Deduction - must, can't, may and might (Verbs)
John has worked all day. He must be tired. (= I assume, believe he is tired)
We use must because we feel 100% certain that John is tired.
Here are some more examples:
– Sarah is a bank manager. She must earn a lot of money.
– Colin hasn't eaten since breakfast. He must feel hungry.
– Harriet isn't at school. She may be ill. (= I'm not sure that she is ill but it is 70% possible)
– Bianca often goes to France. She might have a boyfriend there: (= I am not sure, but it is 40% possible that she has a boyfriend there)
We use could to show that something is possibly true. It is less certain than may and might:
– She could have a boyfriend there.
– He has eaten six hamburgers. He can't be hungry. (= I think it is impossible that he is hungry)
– I don't believe it. It can't be six o'clock! (= I am very surprised)
– The sun is shining but the ground is wet. It must have rained last night.
– He has arrived early. He must have driven very fast.
– The plane is late. It may have been foggy at the airport.
– The money has disappeared. Ted might have stolen it.
– I didn't see Tom at the party. He could have gone home early.
– He hasn't got a driving licence. He can't have driven to London.
– She doesn't know French. She can't have understood the film.
– I don't believe it! Harry can't have escaped from prison.
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