he asked his doctor to tell him whether his operation had been successful, but the doctor

题目

he asked his doctor to tell him whether his operation had been successful, but the doctor
he asked his doctor to tell him (whether) his operation had been successful, but the doctor refused to do so.
he asked (if ) Mr. Gilbert's operation had been successful and the doctor told him that it had been.
then Dr. Millington asked the caller (if ) he was a relative of the patient.
这三个里面分别用了whether 和if, 请帮我解释一下,谢谢 是新概念2册 第39课的内容.

答案

当 (if ) (whether 0引导宾语从句是一般可以通用.