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。

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　　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。