I have no idea _____ he is .填whom还是 who

题目

I have no idea _____ he is .填whom还是 who
可是答案给的 whom 我也是不懂，希望给出解释

答案

who 和 whom,很多人都搞乱的
最主要的差别是 主动 和 被动
who is he
whom he is
看的出不同吗?
在给你一个
who was the man talking to you just now
with whom were you talking to just now