I have no idea____he will come back from work.

问题描述:

I have no idea____he will come back from work.
为什么不填that?

填that表示是that引导的同位语从句,但idea 跟he will come back from work并不是等价的,所以二者并不是同位语;
相应的,idea 跟 whether he will come back from work 是等价的