21.5 must表示推测
 
 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。






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