一、二者区别
1）引导宾语从句时通常可以互相替换, 口语中多用 if 代替 whether.
常放在动词ask 、see 、say 、know和find out 等后面.
Nobody knows whether (if) it will rain tomorrow. 没有人知道明天是否下雨.
Lucy asked whether (if) they had a cotton sweater .露西问他们是否有棉制的羊毛衣.
Ask if/whether she would take the apple.
2） if 引导条件状语从句, "假如；如果"， 而whether引导让步状语从句, "不管；无论" ；
If you ask him, he will help you.
I shall go, whether you come with me or stay at home.
二、只能用whether的情况：
1. 强调两方面的选择, 特别是whether or not连用时.
He didn't say whether or not he'll be staying here.
Let me know whether you can come or not.
但是，当or not 句子后置，也可以用if . (伦道夫. 夸克，1998《朗文英语语法大全》)
He didn't say if he'll be staying here or not.
2. 宾语从句前置时.
Whether this is true, I can't say.
3. 引导主语、表语、同位语从句时.
Whether the football game will be played depends on the weather. （主语从句）
The question is whether he can do it. （表语从句）
The question whether we should go with them must be decided at once. （同位语从句）
4. 在介词之后作介词的宾语时。
I haven't settled the question of whether I'll go back home.
5. 在不定式前与不定式一起组成词组时.
Whether to go or stay is still a question.
6. 在discuss, decide 等动词后作宾语时.
We discussed whether we should go there by plane.
if 可以用来引导一个否定的宾语从句, whether则不能用来引导否定的宾语从句, 因为它表示正反两方面的选择意义比较强.
Tell me if it is not going to rain,please.
请告诉我明天是否不会下雨.
He considered if he shouldn't tell her the secret.
他考虑他是否不该告诉她这个秘密.
三、if或whether引导宾语从句时,要注意三个方面, 即连词、语序和时态.
1） if或whether不能和that 或其它连词（副词）同时使用,也不能省去；
我不知道他今天是否会来.
[误] I don’t know that if(whether) he will come here today .
[正] I don’t know if(whether) he will come here today .
他走过来看看猫是否出了毛病.
[误] He came to see what if (whether) there was wrong with his cat .
[正] He came to see if (whether) there was wrong with his cat .
2） if或whether引导的宾语从句时,虽具有疑问意义,但从句语序应用陈述句语序；
Did you know the way to the hospital ? The old woman asked me .
The old woman asked me if (whether) I knew the way to the hospital .
老妇人问我是否知道去医院的路.
3 ）if或whether引导的宾语从句应和主句的时态保持一致。 即主句为一般现在时, 从句用任一种时态；主句为一般过去时,从句则用过去时中的任一种时态.如：
I don’t know if (whether) he has come here . 我不知道他是否来过这儿.
Tom asked if (whether) I had read the book . 汤姆问我是否看过这本书 .
四、总结
1） 在动词不定式之前只能用whether .如：
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：
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：
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：
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）凡可能引起歧义的地方，有时为了区别是疑问分句还是条件句，有必要用语调或标点来分隔。
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.疑问分句：
I'll tell you later if/whether I can find the time.
条件句：
I'll tell you later, if I can find the time = If I can find the time, I'll tell you later.
所以，用 if 会引起歧义时, 不妨只用whether。