B． cancould have done

　　1．表示对过去事实的推测，常用于否定句和疑问句。   He can't couldn't have watched TV yesterday for he knew they would have an exam. 昨天他不可能看电视，因为他知道快要考试了。   I don't know why she didn't call me yesterday. Can (Could) she have forgotten my telephone number 我不知道昨天她为什么没给我打电话，难道她会把我的电话号码忘了吗？   He didn't come to school yesterday. Can he have been ill 他昨天没来学校，难道他生病了？

　　2．could可表示对过去能做而未做的事感到惋惜、遗憾，可译成“本来是可以……的”。   Why didn't Sophie apply for the job She could have got it. 为什么索菲不申请这项工作？她本来是能得到的。   Why did you stay at a hotel when you went to New York You could have stayed with Barbara.  你到纽约时为什么住宾馆？你完全可以和芭芭拉住在一起的。   Given more time, we could have done the work better. 如果给我们更多的时间，我们完全可以把这项工作干得更好。

　　C． must have done

　　1．表示对过去事实有把握的推测，具有较大的可能性。只用于肯定句中，否定句和疑问句用cancould have done。   His watch must have stopped. 他的表肯定停了。   --- I rang your flat yesterday. A man answered but I didn't recognize the voice. 昨天我给你家打电话，一位男士接了电话，但我没听出来是谁的声音。   --- Oh, it must have been my brother Peter.  哦，那一定是我哥哥彼得。

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