1) must表示必须，否定回答时用needn’t. mustn’t表示不许可。
you must get to the station before 3, otherwise you will miss the train.must i hand in my homework tomorrow?
no, you needn’t. you can turn it in the day after tomorrow.
you mustn’t smoke in the hospital.
she must live near here, for she comes to work on foot.
she was absent from class. there must have been something wrong with her.
the light was out. they must have gone to bed.
mary’s score on the test is the highest in her class. she ____ have studied very hard.(cet-4 1998.1)
d) ought to