1.whether可置于句首引导主语从句,而if不能.
2.whether可用在介词之后,引导宾语从句,而If则不能.
如：Success depends on whether we make evough effort.
3.whether可引导表语从句,if则不能.
如：The puestion is whether it is not worthdoing.
4.在名词之后引导同位语从句时,用whether而不用if.
如：The puestion whether the students answered correctly should be decided by the teacher.
5.whether后可接不定式,而if不能.如：Please tell us whether to go or stay here.
6.在有些动词（如discuss）之后,只能用whether,而不用if引导宾语从句.如：We discussed whether we should close the shop.
7.在引导否定概念in宾语从句时,只能用if,而不用whether.如：he asked me if I hadn't finished my work.