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）；
(2)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
（3）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．
（4）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．
I 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。
It’s time I was going.
It’s time somebody taught you to behave yourself.
7. 由wish引起的表示愿望的虚拟语气
A．用wish表示对现在的愿望时，它所引起的宾语从句中谓语动词形式为：过去式（be动词用were）。
B．用wish表示对将来的愿望时，它所引起的宾语从句中谓语动词形式为：would, could, might+ 动词原形。
C．wish用于对过去的事实表示一种不可能实现的愿望时，宾语从句中的谓语动词形式为：had+动词过去分词或could, would + have +动词过去分词。
I wish that he weren’t so lazy.
I wish I hadn't wasted so much time.
I wish she would change her mind.
8. had hoped/ planned/ thought/ wanted/ intended表示一种过去未实现的愿望或令人失望的事。
I had thought he had been dead for at least 20 years.
I had hoped that she would go to the U.S. and study there, but she said
she liked to stay in China.
9. would rather.
would (had) rather, would as soon, would sooner和would prefer所引起的从句中要求用过去式表示当时或将来的情况，用过去完成时表示过去的情况，表示希望或婉转的责备。例如：
I’d rather you posted the letter right away. 我倒希望你把这封信立刻寄出去。
I would prefer he didn’t stay there too long.我倒希望他不要在那儿可得太久。
I would just as soon you had returned the book yesterday.我真希望你昨天把这本书还了。
10.It’s (high, about) time that 句型中that从句中要求用虚拟语气形式，即动词要用过去式。
It’s time that something was done about the traffic problem downtown.
It is high time that we put an end to this discussion.现在是我们该结束讲座的时候了
11. 虚拟语气在由某些连词或个词引出的表示条件、目的的状语从旬中的使用。常见的这些连词和介词有：
1) unless (除非)，provided(倘若)，supposing(假若)，providing(倘若) , on condition that(如果),由这些词、词组引出的从句，动词多用过去时。
除非你命令他做，要不他是不会做的。
He wouldn't do it unless you ordered him to.
假若你有机会出国，你会去哪儿呢？
Providing you had the opportunity to go abroad, where would you go?
12. so that (以便 )，for fear that(以免, 唯恐)，in order that (以便)，whether(不管)，lest(以免)，in case (假使)，从句中加情态动词，有时should可省略。
他把它藏起来以免她看见。
He hid it lest she (should) see it.
我得准备点啤酒，说不定约翰会来。
I'll get some beer in case John should come.
他轻轻地进屋，以免吵醒他的同房。
He entered the room quietly in order that he should not wake his roommate．
13. if only
如果表示对现在的愿望，句中的谓语动词常用过去式；
如果表示将来的愿望，句中的谓语动词常用would/could+动词原形；
如果表示对过去的愿望，句中的谓语动词常用had+过去分词。
if only I were you!
If only you had come yesterday!
If only Linda could go with us tomorrow!
If only I had said more about it!
only if表示"只有"；if only则表示"如果……就好了"。If only也可用于陈述语气。
I wake up only if the alarm clock rings. 只有闹钟响了，我才会醒。
If only the alarm clock had rung. 当时闹钟响了，就好了。
If only he comes early. 但愿他早点回来。