if和whether的区别 55
主语表语同位语从句都是whether的
If只能在宾语和条件状语
从句中寻求生存
，当“如果”、“否定”
时用if一、绝对用whether
的情况1、引导主语从句
It is unknown whether he will come.
Whether the news is true remains a question.
2、引导表语从句
The question is whether they can take our advice.
3、引导同位语从句
The question whether he'll come is unknown.
二、引导宾语从句
这时两者的含义区别很小，一般可通用。
特殊情况：
1、有or not
存在时，要看是否紧连空格
I don't know whether/if they will come to help us.
whether与or not
粘在一起
连成一个词组时
用whether
I don't know whether or not
they will come for  ...