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1) must用在肯定句中表示较有把握的推测,意为"一定".
2) must表对现在的状态或现在正发生的事情的推测时, must后面通常接系动词 be的原形或行为动词的进行式。
You have worked hard all day.You must be tired. 你辛苦干一整天,一定累了。(对现在情况的推测判断)
He must be working in his office. 他一定在办公室工作呢。
比较:
He must be staying there.
他现在肯定呆在那里。
He must stay there.
他必须呆在那。
3) must表示对已发生的事情的推测时, must要接完成式。
I didn't hear the phone. I must have been asleep. 我刚才没有听到电话,我想必是睡着了。
4) must表示对过去某时正发生的事情的推测, must后面要接不定式的完成进行式。
- - - Why didn't you answer my phone call?
- - - Well, I must have been sleeping, so I didn't hear it.
5)否定推测用 can't.
If Tom didn't leave here until five o'clock, he can't be home yet. 如果汤姆五点才离开这儿,他此时一定还未到家。