Must does not always express compulsion or obligation. It is sometimes used to express inference or possibility or certainty.
- She must be home by now. (= I think she is certainly at home.)
- There is somebody at the door. That must be the postman. (= I think that is certainly the postman.)
- She must be about forty years old. (inference / possibility)
- What he says must be true.
Must can be used with a perfect infinitive (have + past participle) to express conclusions about the past.
- You must have been mad to do such a thing.
- ‘Somebody phoned while you were out.’ ‘That must have been Susan.
Must and should
Note that should can be used as a weaker form of must in this case.
- She must have gone home. (= I think she has certainly gone home.)
- She should have gone home. (= I think she has probably gone home.)
Rewrite the following sentences using must.
1. I suppose you are mistaken.
2. I think he is certainly the oldest man in the village.
3. I think she is older than her husband.
4. Surely he is a fool to behave like that.
5. I suppose the poor fellow was cheated by somebody.
1. You must be mistaken.
2. He must be the oldest man in the village.
3. She must be older than her husband.
4. He must be a fool to behave like that.
5. The poor fellow must have been cheated by somebody.