1） must表示必須，否定回答時用needn't. mustn't表示不許可。
「例如」
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.
2） must用於肯定句，可以表示推測，翻譯為：一定或想必。
後面接動詞的一般時，表示對現在的推測，接動詞的完成式，表示對過去的推測。對過去的推測是考試的重點。
「例如」
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）
a） may
b） should
c） must
d） ought to
“must +不定式完成式”表示對過去發生事情的推測。題意為：瑪麗的考試成績全班第一，她學習一定很刻苦。答案為c.