“She’s here to ask for help,isn’t she?” he asked.这题变成间接引语后就是He asked whether she was there to ask for help or not.这里为什么要用whether or not?可以用if 代替么

问题描述:

“She’s here to ask for help,isn’t she?” he asked.
这题变成间接引语后就是He asked whether she was there to ask for help or not.
这里为什么要用whether or not?可以用if 代替么

whether or not “是否”,固定搭配
可以用if代替,句子变为:
He asked if she was there to ask for help .