1 用于一般疑问句变宾语从句 if 为引导词 意思是“是否”
I want to know if he can come.
2 if 引导条件状语从句 意思是“如果”
If he walks to school at 6:50am ,he will be late for school
3 if引导虚拟语气
If I were you,I would do it well.