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.
