In many grammar books, whenever Who vs Whom comes up, you get the rule (who is used for to refer to the subject, whom is for the object or preposition) and then you get this supposedly handy tip:
If you are confused about using who/whom, try substituting he/him or they/them to see which makes sense.
If he/they makes sense, use who.
If him/them makes sense, use whom.
e.g. [Who/whom] do you serve? You serve him.
Therefore: 'Whom do you serve' =
Aren't there examples where this trick fails to work? Can you think of any?
I ran into this problem applying the trick to the question 'Who are you?'. I know, instinctively, that Who is correct, but if I were to use the trick, I would get it wrong:
[Who/whom] are you? You are him.
Therefore: 'Whom are you' =
But this is obviously
INCORRECT, so what gives? What am I doing wrong? What's the best way to remember how to correctly use Who/Whom