关于虚拟语气的一些问题.

首先,要理解虚拟语气的定义：虚拟语气是一种不肯定的语气或者是假设的语气,比如if(如果,假如),as if（就像,只是像但实际上不是）.
其次,要分清楚虚拟语气的几种常见类型：
一.If引导的非真实条件句（注意if引导的可以实现的条件,称为真实条件句）
If从句 ,主句
（将来） 1.过去式（be只能用were） ,would/should/could/might+V原
2.were to + V原 ,would/should/could/might+V原
3.should+V原 ,would/should/could/might+V原
If there should be no air,there would be no living things.
（现在）过去式（be只能用were） ,would/should/could/might+V原
If I were a bird,I would fly everywhere.
(过去) had done ,would/should/could/might+have done
If we had started earlier,we should not have missed the train.
(当然,有时候是主句在前,if从句在后,规则是一样的.)
二.从句中的虚拟语气
1.表示愿望,建议,命令,要求等的宾语从句中,要用 “ (should) +动词原形” ,这些词有：
wish,suggest,order,demand,propose,command,request,recommend,require,decide,insist,desire,urge,suggestion,proposal,plan,motion,recommendation,demand,order,desire,request,requirement,insistence,advice,decision ,obligatory .
Our teacher suggested that we (should) give the old man some help.
2.在It is/was + 某些形容词 + that 从句的句型中,that从句中的谓语动词常用虚拟语气,即 “should + 动词原形”.这些形容词有:
important,necessary,essential,imperative,natural,strange,advisable,desirable,possible,probable,astonishing,surprising,desired,suggested,requested,recommended,ordered,proposed,decided,moved等
It is important that we should get there on time.
3.在 “ It is (about/ high ) time + that (从句)” 中,谓语动词常用过去式表表示虚拟语气
It is high time that people learnt English.
4.在 “as if/ as though” 引导的状语从句中,谓语动词与wish的宾语从句的虚拟形式相同.
It seems as though it were already spring.
三.省略if的非真实条件句
if省去,将助动词.情态动词.be或have放在句首形成部分倒装.
If I were you,I would not do those stupid things.
= Were I you,I would not do those stupid things.
If he had known about this,he would have been very sad.
= Had he known about this,he would have been very sad.
四.含蓄虚拟语气
有时假设的情况并不以条件从句表示出来,而是暗含在上下文中,比如通过介词短语来表示.But for（要不是） without（没有...的话）otherwise（否则）
But for your advice,I could not have done it so successfully.
The change could not have taken place without the open-door policy.
I was so busy then,Otherwise,I would have told him the answer.
五.错综时间虚拟句
非真实条件句中主句和从句的谓语动词所指时间有时可能指不同的时间,这时要根据上下文的意思采用不同的谓语动词形式.
If I were you,I would have taken his advice.(从句指现在,主句指过去)
If I had taken my raincoat with me this morning,I would not be wet now.(从句指过去,主句指现在).
六.用在if only 引起的感叹句中.
If only the driver didn’t drive so fast!