虚拟语气，又称假设语气，是谓语动词的一种形式，表示说话人叙述的内容与事实相反，在现实中并不存在，或实现的可能性很小。
If I were a bird, I could fly in the air.
A 虚拟语气的三大种类
与现在事实相反
if条件句的谓语 were\ did 主句的谓语 would (could/should/might) + do
If I were you, I should go and try.
I would go if they treated me like a slave.
与过去事实相反
if条件句的谓语 had done
主句的谓语 would (could/should/might) + have done
If it had not rained this morning, I should have gone shopping.
与将来事实相反
if条件句的谓语 were\ did\ should do\ were to do
主句的谓语 would (could/should/might) + do
If he were here tomorrow, I would speak to him.
If she had time, she could help me.
If it should rain, the crops could be saved.
If I were to do the work, I should do it in a different way.
B 省略if的条件句 （用倒装结构）
Were it necessary, I might go without delay. (= If it were necessary,...)
Had you taken my advice, you wouldn't have failed in the exam.
(= If you had taken my advice,...)
C 错综条件句
虚拟条件句中的主句和从句涉及的动作发生的时间不一致。
If you hadn't watched TV yesterday, you wouldn't be so sleepy now.
If you hesitated this moment, you might suffer in the future.
D 含蓄条件句
非真实条件句中的条件从句有时不表达出来，只暗含在上下文中，这种句子叫做含蓄条件句。
He must have been there, or he never could know the place so well.
Such mistakes could have been avoided.
E 虚拟语气在从句中的用法
主句中的动词、名词、形容词如表示要求、建议、命令或愿望等含义，与其相关的名词性从句(主语从句、宾语从句、表语从句和同位语从句)中的谓语动词要用虚拟语气。
It is suggested that each student (should) sing a song in English.
The workers demanded that their wages (should) be raised by 10 percent.
My advice is that you (should) practice speaking English as often as possible.
在It is (high) time (that) ...结构中，定语从句的谓语动词用过去式，表示“到某人该做某事的时间了”。
It is（high） time that the children went to bed.
The teacher treats the pupil as if he were her own child. （方式状语从句）
I felt as though we had known each other for years. （方式状语从句）
She emphasized it again and again, lest he (should) forget. （目的状语从句）
Though he (should) fail, there would still be hope. （让步状语从句）
F 其它句型中的虚拟语气
1, if only...(如果…就好了) If only you had told me the truth before.
2, would rather （宁愿） I would rather I hadn't seen that film yesterday
