4) 表示现在和将来情况的虚拟条件句：
⑴表示现在或将来的虚拟条件句——从句中的谓语(用过去式形式);主句谓语(would+动词原形)
If I had a map I would lend it to you.
If you left your bike outside, someone would steal it.
If I were you I’d plant some trees round the house.
注：If I were you句中，were不能改成was,但在第三人称单数后用was也可以。如：
If Helen were(was) here how nice it would be!
⑵主句谓语有时也可以由might, could构成。
If I could type, I might save a lot of time.
If he had a permit he could get a job.
⑶在条件句从句中有时还可以用were to +动词，或should+动词。
If he were (was) to resign, who would take his place?
If I should be free tomorrow, I will come to see you.
⑷在某些条件从句中(如were to +动词，should+动词)，if有时可以省略，这时语序要改为倒装。
Were I Tom, I would refuse.
Were I in your shoes I would let him know what I thought of him.
Should she be interested, I’ll phone her.
⑸从句有时可以用If it were not for这种句型，表示“若不是”。
If it were not for your help, we would be in serious trouble.
5) 表示过去情况的虚拟条件句：(这类从句表示一个与事实相反的过去情况)
⑴表示过去情况的虚拟条件句——条件从句中的谓语(相当于过去式的形式);主句的谓语(would have+过去分词)
If I had known that you were coming, I would have met you at the airport.
If I had left sooner, I’d have caught the bus.
What would you have done if you’d been in my position?
注：在主句中，如果主语是第一人称，可以用should构成谓语。
I should (would) have talked it to my mother if I had thought of it.
⑵主句中也可用could和might构成，有时还可由should构成。
If my father had lived, he could have done all this for me.
If we had found him earlier we might have saved his life.
If he had apologized, you should have done so too.
⑶从句中也可用had not been for来构成谓语，表示“要不是”。
If it hadn’t been for your timely help, we would have got into serious trouble. 考试大
If it hadn’t been for her meticulous care, I wouldn’t have got well so soon.
⑷if有时可以省略，但后面部分需要倒装。
Had I not seen it with my own eyes, I would not have believed it.
Had it not been for their support, we couldn’t have won the election.
Had Mark been in charge, it wouldn’t have happened.
