与过去事实相反：从句的谓语动词用had done，主句谓语动词用would/could/might/should have done；与现在事实相反：从句的谓语动词用过去式，主句谓语动词形式用would/could/might/should do；与将来事实相反：从句的谓语动词用过去式should do/were to do，主句谓语动词形式用would/could/might/should do。
If引导的条件状语从句中的虚拟语气
表示对过去情况的虚拟：
从句：had done
主句：would/could/might/should have done
If I had read the book,I would have told you about it.
如果我读了这本书，我就会告诉你了。
表示对现在情况的虚拟：
从句：would/could/might/should do
主句：动词过去式
If he studied at this school，he would know the environment around the school very much.
如果他在这所学校学习的话，他会很了解周边环境。
表示对将来情况的虚拟：
从句：should do/were to do
主句：would/could/might/should do
If he should go to Harvard University,he would make full use of his time.
如果他要上哈佛大学的话，他就会充分利用他的时间了。
