A．may
/might /have done
1．表示对过去事实不大肯定的推测，常译为“可能已经”。
They
might have taken a later train.也许他们乘坐的是晚一点儿的火车。
He seems
to know the city quite well. He may have been there before.他似乎对那个城市很熟悉，他以前可能去过那儿。
2．表示“本来应该或可以做但没有做某事”，含有轻微的责备口吻或遗憾。
You
might have given him more help, though you were busy.你本来应该给他更多的帮助，虽然你很忙。
The
proposal might have been refused.这个建议本该拒绝的。
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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