if引导的从句3种情况:
1.条件状语从句，译为“如果”。
结构：if+一般现在时，主语+将来时
If you ask him, he will help you.
He will come if you invite him.
2.宾语从句，译为“是否”。
I wonder if she is ill.
不知她是否病了。
I want to know if he lives there.
我想知道他是否住在那儿。
He asked me if I could help him. 他问我是否能帮他。
3.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.
