英语六级基础语法知识：情态动词must
1) must表示必须，否定回答时用needn’t. mustn’t表示不许可。
【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用于肯定句，可以表示推测，翻译为：一定或想必。后面接动词的一般时，表示对现在的推测，接动词的完成式，表示对过去的推测。对过去的推测是考试的重点。
【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"表示对过去发生事情的推测。题意为：玛丽的考试成绩全班第一，她学习一定很刻苦。答案为c.
From:https://www.zgghmh.com/CET-6/20007640.asp