OK,知识很多,我先占个楼,慢慢讲.
1,whether和if常用来引导宾语从句,这时两者的含义区别很小,一般可通用.（这里需要注意的是：如果是whether.or not的搭配,那就不能通用.）
例子：
I don't know whether/if they will come to help us.
2,只能用whether的情况：
A：在介词后面
（This depends upon whether we are determined to do it.）
B：动词不定式之前
（She doesn't knows whether to get married now or wait.）
C：在及物动词后的宾语从句中（We discussed whether we should make a change in our plan.）
D：引导主语从句
（Whether the news is true remains a question.）
E：引导表语从句
（What we want to know is whether he will come to speak to us tomorrow.）
F：引导同位语从句,常用whether
（The question whether we'll build another lecture building hasn't been settled.）
小结一下上面的：其中CDEF条合起来统称名词性从句需要用whether
3,只能用if的情况：
A：表否定的宾语从句
（He considered if he shouldn't tell her the secret.）