1．虚拟语气在条件句中的用法
（1）若表示与现在事实相反的假设,则条件从句中用过去式,主句中用过去将来式（would,should,could,might + 动词原形）；若表示某事将来实现的可能性不大,则条件从句中用should + 动词原形也可用“were to+动词原形”或用过去式动词.如：
I would certainly go if I had time．（现在）
If it should rain,we wouldn”t go out.（将来）
If I knew German,I would read the paper to you．（现在）
If he were here,we could ask him．（现在）
（2）若表示与过去事实相反的假设,从句中用过去完成式,主句中用过去将来完成式（should,would,could,might + have + 过去分词）.如：
If I had set off a little earlier,I would have caught the train．
She would / might have come if she hadn”t been so busy．
If I hadn”t taken your advice,I would have made a bad mistake．
If only you hadn”t told him the news．
（3）含有虚拟语气的条件状语从句中,如有had,should,were 这三个词的话,在正式或书面语言中可将if省略,再将句子的主语和谓语动词实行全部倒装或部分倒装.如：
Had they time（=If they had time）,they would certainly come and help us．
Should it be fine（=If it should be fine）,we would go for an outing．
Were I go（=If I were you）,I would go．
（4）若主句从句所指的时间不一致,即条件从句表示与过去事实相反,主句表示与现在事实相反；或者条件从句表示与现在事实相反,主句表真实情况,则从句中应采用与具体时间相对应的虚拟形式.如：
If it had rained last night（过去）,It would be very cold today（现在）．
If I had taken your advice（过去）,I should have no trouble with the work now（现在）．
2．虚拟语气在宾语从句中的用法
在动词suggest,order,demand,propose,request,command,insist等后的宾语从句中,用虚拟语气（即Should+动词原形或只用动词原形）来表示愿望、建议、命令、请求等.在动词wish /would rather后的宾语从句中,用过去式表示与现在事实不符,用过去完成式表示与过去事实不符.如：
I suggest that we（should）set off at once．
The doctor insisted that the patient (should)be X-rayed．
I wish I were as strong as you．
would rather that you hadn”t told him．
3．虚拟语气在主语从句中的用法
在It is necessary/important/strange/natural；It is requested/suggested/desired/proposed；it is a pity等结构后的主语从句中要用虚拟语气,即Should+动词原形或只用动词原形.如：
It is necessary that he（should）be sent there at once．
It is requested that Professor Li（should）give us a speech．
It is desired that we（should）get everything ready by tonight．
4．虚拟语气在表语从句、同位语从句中的用法.
在 suggestion,proposal,idea,plan,order,advice等名词后的表语从句、同位语从句中要用虚拟语气,即should+动词原形或只用动词原形.如：
Our suggestion is that you（ should）be the first to go．
My advice is that we（should）send for Doctor Li．
Do you know the order that you（should）keep watch?
5．虚拟语气在其他状语从句中的用法
在as if引导的状语从句中,用过去式表示与现在事实不符,用过去完成式表示与过去事实不符；在so that,in order that引导的状语从句中,常用should/would/could/might+动词原形来表示虚拟语气.如：
They talked as if they had been friends for years．
She stayed at home for a few days so that she could take care of her mother．
He took a taxi to the station so that she should not miss the train．
6．虚拟语气在定语从句中的用法：
在it is time后面的定语从句中常用过去式表示虚拟
