注意： (1)若条件句中有were，had，should，可把if 省略，而把were，had，should 放在主语前，形成倒装结构，这种结构主要用于书面语中。
Had you taken my advice，you wouldn’t have failed in the examination.如果你听了我的建议，你就不会考试不及格了。
(2)有时主句和虚拟条件从句的动作发生的时间不一致，此时，主句和从句的谓语动词要根据各自所指的不同时间选择适当的动词形式。
If you had worked hard，you would be very tired now.
