whether和if都可以引导宾语从句,表示“是否”,在口语或间接引语中两者可以互换使用.如：
I wonder if / whether I can get some advice from you.
Ask him whether / if he can come.
但在有些情况下,whether和if的用法有一定区别.
1.whether引导的从句常可以与连词or或or not直接连用,而if一般不能.如：
正：Let me know whether you can come or not.
误：Let me know if you can come or not.
2.当宾语从句提到句首时,只能用whether引导,而不能用if.如：
正：Whether it is true or not,I can’t tell.
误：If it is true or not,I can’t tell.
3.whether可以引导带to的不定式,if则不能.如：
正：I don’t know whether to accept or refuse.
误：I don’t know if to accept or refuse.
4.whether及其引导的成分可放于介词之后,作介词的宾语,但if不能.如：
正：I worry about whether I hurt her feelings.
误：I worry about if I hurt her feelings.
5.whether可以引导从句,作主语、表语或同位语,而if不能.如：
正：It was uncertain whether he would come.
误：It was uncertain if he would come.
正：His first question was whether Tom had arrived yet.
误：His first question was if Tom had arrived yet.
正：We must consider the question whether we will take these measures.
误：We must consider the question if we will take these measures.