if引导的虚拟语句的时态

　　2. 当if后面接一般过去时的动词时,则是一种虚拟语气.

　　if sb did sth sb would/should do sth.

　　If I didn't make that mistake, I would't have suffered such a failure.

　　3. 当if后面接什么时态都可以时，if为“是否”的意思，句子为宾语从句.

　　sb do sth if sb do sth.

　　He said if rain was stop.

　　由if引导的条件状语从句，时态应该是主将从现，就是说主句用一般将来时，而从句用一般现在时表示将来。

　　例如 If it doesn't rain tomorrow, I will visit my former school.

　　这里从句就用一般现在时。

　　当然if引导的从句也可以用虚拟语气

　　例如If I got up early in the morning, I wouldn't miss the train.