引导的三种从句
一是条件状语从句，译为“如果”。
He will come if you invite him. 如果你请他，他会来的。
二是宾语从句，译为“是否”。
I wonder if she is ill. 不知她是否病了。
三是引导非真实条件句，使用虚拟语气，译为“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。
拓展
一、由when , as soon as , not until 等引导的时间状语从句也是遵循“主将从现”原则。
如：I will tell him about it as soon as he comes back.
他一回来我就告诉他这件事。
二、if 也可引导宾语从句，意思是”是否“
如： I don't know if he will come tomorrow.
我不知道他明天是否回来。
