这个问题很好。who在引导宾语从句时可以在从句中做主语，也可以代替whom做宾语表语。因此会出现两种情况。当作主语时是who is当做宾语表语时是who he is.I don't know who is his dad.I don't know who he is.I don't know who loves him.I don't know whom(who) he loves.
