關聯詞只能用whether不能用if表示“是否”的情況如下:
A)在表語從句和同位語從句中。例如：
The question is whether the film is worth seeing.
The news whether our team has won the match is unknown.
B)在主語從句中,只有用it作形式主語時,whether和if都能引導主語從句,否則,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)在介詞之后。(介詞往往可以省略)例如：
It all depends (on) whether they will support us.
D)后面直接跟動詞不定式時。
He doesn't know whether to stay or not.
E)后面緊接or not 時。
We didn't know whether or not she was ready.
F)引導讓步狀語從句,只能用whether。
Whether you like it or not, you must do it well.
G)用if會引起歧義時。例如：Please let me know if you like it.
該句有兩個意思：“請告訴我你是否喜歡”。
或“如果你喜歡,請告訴我。”用了whether就可以避免。