关联词只能用whether不能用if表示“是否”的情况如下:
A)在表语从句和同位语从句中。例如：
The question is whether the film is worth seeing.
The news whether our team has won the match is unknown.
B)在主语从句中,只有用it作形式主语时,whether和if都能引导主语从句,否则,也只能用whether。例如：
Whether we shall attend the meeting hasn't been decided yet.
It hasn't been decided whether(if)we shall attend the meeting.
C)在介词之后。(介词往往可以省略)例如：
It all depends (on) whether they will support us.
D)后面直接跟动词不定式时。
He doesn't know whether to stay or not.
E)后面紧接or not 时。
We didn't know whether or not she was ready.
F)引导让步状语从句,只能用whether。
Whether you like it or not, you must do it well.
G)用if会引起歧义时。例如：Please let me know if you like it.
该句有两个意思：“请告诉我你是否喜欢”。
或“如果你喜欢,请告诉我。”用了whether就可以避免。