could have done,should have done ,must have done,的区别是什么...

本来能做成的
should have done将要完成 ,must have done必须完成

should have done 本可以做
I should have done the work better
我本可以把这项工作做得更好。
must have done 一定已经做了
He must have done it.
他一定已经把这事做了
They must have done it!
这件事一定是他们干的

should have done,must have done都表示该做的没做,是虚拟语气,用于表达与过去的事实相反.但should have done语气上没有must have done重.它们有细微的差别.给你2个句子比较一下:
1)You should not have told him such a bad news when he was ill.
2)You mustn't have told him such a bad news when he was ill.
这2句话翻译过来可以是一样的,但是第1句的含义是,虽然你这么做错了,但是说话者并没有责怪你,责备你的意思,只是叫你注意,以后可不能这么做了.
而第2句话就可以是在责备你为什么要这么做,设想,如果说话者本来平时就和你关系不太好,第2句话甚至可以可以让你难堪,下不了台,对你进行报复.
总之,细微的差别就在语气之中.虚拟语气是英语学习的重难点,多加油哦!

虚拟语气
表应该完成却没能完成