In the sentence:
Bob does not know who/whom the thief is.
Should it be who or whom? and why? I don't really understand it in cases like this. I was told to think about them being analogous to him/he ...
I have a grammar which says that "whom" is used when it follow a preposition.
E.g: to whom am I speaking. to whom it may concern.
The grammar also says that "whom" is the object form of "who".