Do you know who will come tomorrow?
（who是主语）
I don't know who is his father.这句话有点怪.
我们一般可以理解为：his father is who的.
Who are you?这个who是表语的.
习惯认为who还是表语,所以下面的句子正确的：
I don't know who his father is.（who是表语）
He wanted to know what had happened the night before.（what主）
判断方法：
改成特殊疑问句,没有“助动词+主语+谓语”结构的,或者说是what,who,which后面直接加谓语.
如：
Who will come?
What happened...
which is right...