1、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。
2、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。
3、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。
if的用法
1、if作为连词，引导条件状语从句，表示“假如，如果”等。在复合句中如果主句用将来时，则if引导的状语从句用一般现在时。
eg:If it rains(从句) ,I will stay at home（主句）。
=I will stay at home if it rains.
He will come if you invite him.
如果你请他,他会来的。
2、主句含有情态动词 （must、may、can etc.）从句为一般现在时。
eg:If you drive too fast ,you may have an accident.
=You may have an accident if you drive too fast.