if引导两种条件句：真实条件句和虚拟条件句.
真实条件句：
If he has written the letter,I'll post it.
If I have time(表示条件的是从句),I'll come over to see you.（由条件导致的结果就是主句）
在真实条件句里,从句用一般现在时表示将来时,主句用一般将来时.
虚拟条件句：
If I had enough money（从句）,I would buy a computer（主句）.
如果我有钱,我要买一台电脑.（但我没有）
虚拟语气用于表达不可能实现的事情.虚拟条件句又有现在情况,将来情况,和过去情况.
1）表示现在或将来：
条件从句中的谓语是相当于过去时的形式,主语谓语用would+动词原形：
If I had a map I would lend it to you.如果我有地图我就借给你.（但我没有）
If I dyed my hair blue everyone would laugh at me.如果我把头发染成蓝色,谁都会笑我的.（当然我不会干这样的傻事）
2）表示过去：
条件从句中的谓语用过去完成时,主句的谓语动词要用would have＋过去分词 ：
If I had known that you were coming,I would have met you at the airport.如果我知道你要来,我就去机场接你了.
If anything had happened,he would have let us know.如果发生了什么情况,他在就通知我们了.