if或whether在引导宾语从句中作“是否”讲,常用在ask,care,wonder,know,等动词后引出带有疑问意义的宾语从句.从句语序保持陈述语序,if或whether不充当成分,只起连接作用,不可省.
区别：whether可以引导介词后的宾语从句,if不可以.
I worry about whether I hurt her feeling.
I wonder if/whether he can come tomorrow.