if引导条件状语从句，遵循主将从现原则，if后是从句。
if 做连词时：
1.(表示条件)如果 He will come if you invite him. 如果你请他,他会来的。
2.(表示虚拟)假如,要是 If I were you, I would never do that. 如果我是你,我决不做那件事。
3.(表示让步)即使 We'll go even if it rains. 即使下雨我们也要去。
4.是否 I wonder if she is ill. 不知她是否病了。
5.(表示因果关系)每一次...的时候(总是会) If I feel any doubt, I inquire. 我一感到有疑问就随时询问。
从句位置：
从句可放在主句前面，此时要用逗号分开，也可放在主句后面。
如：
1、If it doesn't rain tomorrow , we will go shopping .（if it doesn't rain tomorrow是从句）
= We will go shopping if it doesn't rain tomorrow.
如果明天不下雨，我们就去购物。
2、He will come if you invite him. (if you invite him 是从句）=If you invite him , he will come . 如果你请他,他会来的。
