1.could have done 表示“本来能做但却没做”。
couldn’t have done 多用于语气强烈的否定,意为“过去不可能做过”
eg：
（1） He could have passed the exam,but he was too careless.
他本来能够及格的，但是他太粗心了。
（2）The accident could have been avoided;the driver couldn’t have slowed down.
这事故本来是可以避免的;司机肯定没减速。
2.may/might have done 意为“可能做过”。
might 所表示的可能性比较弱
might have done 还可表示“本可能做而实际上未做”,含有轻微的责备语气。
eg：
You might have given him more help,though you were busy with your work.
你本来可以给他更多的帮助,尽管你工作很忙。
3.must have done 意为“（过去）一定做过”,只用于肯定句。
eg：It must have rained last night,for the ground is wet.
昨晚一定下雨了,因为地面是湿的。
4.can't have done 意为“(过去)不可能做过”
eg：Mr smith can't have have gone to Beijing,for I saw him in the library just now.
5.should/ought to have done “本该做某事而实际上未做”;
should not have done/ ought not have done 表示“不该做某事反而做了”。
eg: Look, Mary is crying.I should'nt /oughtn't have spoken to her so rudely.
6.needn’t have done “本来不必做某事而实际上却做了”。
eg: I got up early, but I needn’t have done so, because I had no work to do that morning.
我起床起得很早，但我本来不必起那么早的，因为我那天早上没什么事做。
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