引導的三種從句
一是條件狀語從句，譯為“如果”。
He will come if you invite him. 如果你請他，他會來的。
二是賓語從句，譯為“是否”。
I wonder if she is ill. 不知她是否病了。
三是引導非真實條件句，使用虛擬語氣，譯為“假如,要是”。
If I were you, I would never do that.
如果我是你，我決不做那件事。
拓展
一、由when , as soon as , not until 等引導的時間狀語從句也是遵循“主將從現”原則。
如：I will tell him about it as soon as he comes back.
他一回來我就告訴他這件事。
二、if 也可引導賓語從句，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來。