1.(表示条件)如果
He will come if you invite him.
意思：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.
意思：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.
意思：即使下雨我们也要去.
4.是否
I wonder if she is ill.
意思：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.
意思：我一感到有疑问就随时询问.