用来表示说的话不是事实，或者是不可能发生的情况，而是一种愿望、建议、假设的语气叫虚拟语气。
一、三种时态的虚拟语气
1. 条件与现在事实相反
从句谓语用一般过去时(be 多用 were)
主句谓语用 would / should / could / might＋动词原形
If I were you, I should accept the invitation.
If I had time, I would go there.
2. 条件与将来事实相反
从句谓语用一般过去时(be 多用 were)或 should ＋动词原形 / were to do
If it should rain / were to rain / rained tomorrow, the sports meet would be put off.
If you dropped / should drop / were to drop the glass, it would break.
3. 条件与过去事实相反
从句谓语用 had done
主句谓语用 would / should / could / might＋have done
If you had come earlier, you couldn"t / wouldn"t have missed the bus.
If he had seen you yesterday, he would have returned your book
二、含蓄虚拟句
(即含有 without / but for, otherwise /
Human beings would die without water.
I couldn"t have succeeded but for your help.
【注意】含有 without / but for，otherwise /主从句中主句的谓语结构。
or 等词的句子)
or 等词的句子谓语结构等同于 if
三、含有 had，should，were 这三个词的虚拟条件状语从句
Had they had time(＝If they had had time)，
they would have certainly come to help us.
Should it be fine(＝If it should be fine)，we would go for an outing.
Were I you(＝If I were you), I would go.
【注意】含有 had，should，were 这三个词的虚拟条件状语从句，在正式或书面
语言中可将 if 省略，把 had，should，were 提前放到句首，变为部分倒装。
四、if 混合虚拟条件从句(即主从句分别与不同的事实相反)
If the weather had been finer, the crops would be growing still better.
If you had followed the teacher"s advice, you wouldn"t be in hospital.
【注意】if 假设条件从句谓语动词发生的时间与主句所假设的谓语动词不一致，
叫做错综条件虚拟语气。主句和从句的谓语动词要依照假设的时间而定。
五、wish 的虚拟语气
从句用一般过去时(did 或 were)
从句用过去完成时(had done)
would/ could/might+do
现在的愿望
过去愿望
将来愿望
How he wishes that he were a bird.
I wish I had seen the film last night.
We wish we would live on the moon one day.
六、would rather 的虚拟语气
从句用一般过去时
表示愿望与现在事实相反
从句用过去完成时
表示愿望与过去事实相反
I would rather you were not here with me now.
I would rather you hadn"t done that.
七、as if / though ＋ 从句中谓语用相应的过去时态(等同于 wish从句用法)
She speaks English as though she were a native of New York.
八、if only (要是……就好了)，句中谓语用相应的过去时态(等同于 wish 从句用法)。
If only you hadn"t told me the news.
九、口诀必背虚拟语气
一个坚持：insist
两个命令：order, command (名词是 order, recommendation)
三个建议：advise, suggest，propose(名词是 advice, suggestion, proposal)
四个要求：demand, require, request, desire(名词是
demand, requirement, request, desire)
从句中所用的虚拟语气结构为 should ＋动词原形，should 可以省略。
The doctor suggests that I (should) exercise every day.
