From: kechpaja <[log in to unmask]>

> Although, if 'who' were being treated as a third person pronoun,
> wouldn't it be "Who is you?" and "Who is I?", which it obviously
> isn't?

It's treated as third person, but it's part of the predicate, whereas the verb agrees only with the subject. The subject is "you" or "I" in such a sentence — "who" is a predicate nominative. 

At last!! Someone other than me who remembers 6th grade grammar.........or may a little Chomskyan "subject-verb question transformation"..... English, like most (all?) the Germanic languages, inverts the subject and verb in questions:

Q-I am a linguist > Am I a linguist?

Then there's the requirement that the Q-word has to come first, so--

Q-you are who > who are you? The Q-word was originally a predicate nominative, as Kechpaja says.

If the element being questioned is actually the direct object of a trans.verb, then the q-word comes first rule applies:

John killed Q-[someone]:  who(m) did John kill?  otherwise:

Q-someone killed John > either who killed John // or Passive: John was killed by whom? or by whom was John killed?

Same with "what": you are eating Q-[something] > what are you eating or what is being eaten by you?

Is that clear now? Class dismissed :-)))))))))))))