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We use must to assert what we infer or conclude to be the most likely interpretation of a situation or events: He must be at least sixty. We don't know for a fact that this is true, but taking everything into account, we think that it is almost certainly so. The opposite of must in this sense is can't: John: He must be at least sixty. Peter: Oh no! He can't be anything like as old as that. John is almost certain that 'He' is sixty, and Peter is equally certain that He' is not. In reported speech, this dialogue becomes: John said he must be at least sixty, but Peter thought he could n't be anything like as old as that. To refer to past time, we use must and can't (or couldn't) with the perfect infinitive: It must have been a great shock to him. ( It was a great shock, I feel sure.) You can't ( or couldn't) have understood what he said. (It seems clear that you didn't understand.) |