whether 和 if区别
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:
    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.
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.
    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.
    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.
    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。
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