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.