whether和if的用法区别
whether与if作“是否”讲，在用法上有相同之处，也有不同之处。
一、相同之处。
1.whether和if都能引导宾语从句，常置于see,ask,learn,tell,wonder,doubt,find out等动词之后。
如:He asked me whether/is I could help him. I want to know whether/if he lives there.
2.whether和if都可用it作形式主语从句中。如: it's uncertain whether/if he'll come this evening.
3.在be uncertain/doubtful之后，常用whether引导宾语从句，有时也可以用if，如: We are uncertain whether/if Mary will come.
二、不同之处。
1.whether可置于句首引导主语从句，而if不能。如: ___the 2000 Olympic Games will be in Beijing is not known yet. A.whether B.If C.Whether D.That 答案是C。
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.
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