01
虚拟语气在非真实条件句中
表示与现在事实相反的虚拟语气，if条件句使用一般过去时（did/were），主句使用过去将来时（would do/be）
If I were you, I would study hard.
If we had time, we would attend the meeting.
表示与过去事实相反的虚拟语气，if条件句使用过去完成时（had done/been），主句使用过去将来完成时（would have done/been）
If I hadn’t been ill, I would have seen you.
If I had known your telephone number then, I would have called you.
表示与将来事实相反的虚拟语气，if条件句使用过去将来时（should/were to），主句使用过去将来时（would do/be）
If it should rain, the crops would be saved.
If it were to snow tomorrow, they would not go out.
以上就是虚拟语气的三种主要形式：对现在、将来、过去进行虚拟。
02
省略if的虚拟语气
虚拟条件句中的if在一些情况下可以省略，请看下面的例子：
If it should happen, what would you do?
Should it happen, what would you do?
If he had recognized me, he would have come over.
Had he recognized me, he would have come over.
If I were in school again, I would work harder.
Were I in school again, I would work harder.
小结：如果从句中含有should 、had、were 时，则可以把这三个词置于句首，省略if，采用倒装语序。记忆小窍门：一马当先“还属我”。
03
混合/错综型虚拟语气
当条件状语从句表示的行为和主句表示的行为所发生的时间不一致时，称为混合虚拟语气或错综虚拟语气，动词的形式要根据它所表示的时间作出相应的调整。
主句是过去，从句是现在
If you had followed my advice, you would be better now.
If you had told me the news yesterday, I wouldn’t be so worried now.
从句是过去，主句是现在
If I were you, I would have taken his advice.
If I were you, I wouldn’t have missed the concert last night.
小结：混合虚拟表示，一半用过去的虚拟，一半用现在的虚拟。
04
情态动词
“情态动词should/ought to/could/may/might +have done”表示“过去本……而事实上未……”的虚拟语气，一般表示委婉的责备。
You are late again. You could have come earlier.
You should have told me earlier that you weren’t able to come
