是whether 和 if
一、whether和if都可以用作连词,引导一个名词性从句,表示“是否”.whether可以引导主语从句、表语从句、宾语从句和同位语从句；而if只能引导宾语从句.
1．引导宾语从句既可以用whether,也可以用if,两者常可以互换.whether从正反两个方面提出疑问,选择意义较强,多用于正式文体中；而if多用于口语,比较强调单方面,如：
He asked me whether / if I could lend him my bike.
Could you tell us whether / if it snows in winter in Australia?
I want to know whether / if you can come or not.
She asked them whether / if they had gone there by car or by train.
值得注意的是：
① whether 和if都可以和or not连用,但是whether之后可以直接跟or not,而if则不能,如：
I don’t care whether or not your car breaks down.
I don’t know whether or not he’s coming.
但是不可以说：
I don’t care if or not your car breaks down.
② whether引导的从句一般不可以为否定结构（选择问句的后一部分除外）；而if则可以跟否定结构,如：
I don’t care if she doesn’t come.
I don’t care if it doesn’t rain.
但是,在某些动词或形容词之后,则可以用whether引导一个否定结构的句子,这种结构往往表示肯定的含义,如：
I wonder whether he doesn’t think too much of himself.
其意思相当于：I think he does think too much of himself.
③ 在个别动词之后,如discuss 等,只跟用whether引导的宾语从句,如：
They discussed whether they should close the shop.
We discussed whether Mary could join us.
④ whether 可以引导从句作介词的宾语,而if则不能,如：
I’m not interested in whether they’ll go or not.
It depends on whether we have got enough money.
⑤ whether可以直接跟动词不定式连用,而if则不能,如：
I didn’t know whether to laugh or to cry.
She hasn’t decided whether to go or not.
He was wondering whether to go home or stay at school.
2．引导主语从句（包括从句在句首的位置）应当用whether,不用if,如：
Whether it is true remains a problem.
Whether he can stay with us is another matter.
Whether he will come, I am not sure.
Whether or not he did it, I cannot tell.
3．引导表语从句用whether,不用if,如：
The question is whether you should accept it.
The question is whether he will speak at the meeting.
4．引导同位语从句用whether,不用if,如：
The question whether he’ll attend the meeting is essential.
I am in doubt whether I should agree to the plan.
二、whether 还可以引导让步状语从句,意为“不管；无论”；而if则不能引导这种从句,如：
I shall tell her about it whether you agree or not.
Whether or not the figures are accurate, we have to recheck them.
Whether or not we had a baby, we could not survive just on my salary.
三、if还可以引导条件状语从句,意为“如果；假如”,而whether则不能引导这种从句,如：
We shall do this experiment if we have time.
四、如果在从句中使用if表示“是否”容易与表示条件的状语从句混淆时,应注意避免使用if,例如：Please let us know if they are coming. 这句话可以理解为“如果他们要来,请通知我们一下.”也可以理解为“请告诉我们,他们是否要来.”若表示后一层意思,应当使用,以免引起误会.
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jiangxl | Lv13 | Answer Date：2009.10.21 16:11 (0) | (0) 1. 在动词不定式之前只能用whether .如：
例 I can’t decide whether to stay. 我不能决定是否留下.
2 在whether …… or not 的固定搭配中.如：
例 I want to know whether it’s good news or not . 我想知道是否是好消息.
3 在介词后,只能用whether.如：
例 His father is worried about whether he lose his work . 他的父亲担心是否会失去工作.
4宾语从句放在句首表示强调时,只能用whether .如：
例whether they can finish the work on time is still a problem . 他们是否能准时完成这项工作还是个问题.
5用if会引起歧义时,只用whether.如：
例 Could you tell me if you know the answer ?
这句话有两种意思：“你能告诉我是否知道答案吗?”或“如果你知道答案,请告诉我,好吗?”.如用whether可避免歧义
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