虚拟:should have done ,could have done请教虚拟语气的几种情况是在什么时候使用的:should have done 2.could have done 3.would have done(是虚拟吗?) 4.might have done(是虚拟的用法吗?) 比如:Mary couldn't have recei

虚拟:should have done ,could have done请教虚拟语气的几种情况是在什么时候使用的:should have done 2.could have done 3.would have done(是虚拟吗?) 4.might have done(是虚拟的用法吗?) 比如:Mary couldn't have recei
虚拟:should have done ,could have done
请教虚拟语气的几种情况是在什么时候使用的:
should have done    2.could have done 3.would have done(是虚拟吗?)  4.might have done(是虚拟的用法吗?)
 
比如:Mary couldn't have received my letter,otherwise she( should/could/would?用哪一个,怎么判别) have replied last week. 
 
另外(should可省略) do sth,是什么情况下用的虚拟语气?
 
我想简单了解一下考试中常考到的虚拟语气的情况就可以了,先不了解全部的虚拟语气的使用情况了. 
比如:Nancy is (supposed) to (have finished) chemical experiment at least two weeks ago.
suppose要用虚拟吗?to后面是do sth,这里用have finished 是说明完成时,还是跟虚拟有关?
虚拟到底是说明与实际情况相反,或者不可能发生的情况下用?还是真实的建议,推测的情况下也用虚拟?

虚拟:should have done ,could have done请教虚拟语气的几种情况是在什么时候使用的:should have done 2.could have done 3.would have done(是虚拟吗?) 4.might have done(是虚拟的用法吗?) 比如:Mary couldn't have recei
虚拟一般是对实际情况的相反,表示假如这么做了或者没有这么做,又会怎么样.像should/could/would have done 都可以用在虚拟语气中,比较常见.而might have done 一般较少用在虚拟语气中.
Mary couldn't have received my letter ,otherwise she would have replied last week. 在一般情况下当我们确定是虚拟语气了,我们可以根据句意来判断用哪个,它们的意思都不一样,should表示应该,could表示可能,would表示会,这句话的意思是Mary不能已经收到我的信,否则她上个星期就会回复我了.在这个句子中,因为有otherwise,还有前面的couldn't have done,我们可以判断它是虚拟语句.
关于should的省略问题,当你确定这句话是一个虚拟语气时,并且它所需要填的词是should时,后面跟上的不是完成时have done,而是动词时,should就可以省略,比如：If I were you,I (should) be .should就可以省略了.
判断虚拟语句的关键还是要知道这句话的意思是什么,不是说见到一些用在虚拟语气中的单词就认定它都是这个用法,句意很重要.问题补充的那句话不是虚拟语气,并没有要表达与实际情况相反,而只是在陈述一件事情,所以不是虚拟,是完成时.
另外,例如suggest 、suppose 等一些词都是有虚拟语气的情况的,应该都会碰到的,到时你可以再加深了解记忆.