If you are asking a question, it's Are you Example: Are you from America? Are you happy? Notes: -This is for questions answerable by YES or NO But, we also use 'is you' on some cases like... Example: My bestfriend is you. Her partner is you. Notes: - 'is you' comes after predicates (words or phrases that modify or tell something about the subject) I've tried my best to use simple English words to explain. I hope it helps. :)

"Which one are you" The subject of the sentence is "you" so the verb is "are". "One" is the direct object so it doesn't determine the verb inflection.

'Which one are you' is the right answer)

Surely no one actually believe that 'which one is you' is correct?!?!? To put it politely, that's what is called non-standard' English.

It depend when you want to use them, but in the mainly cases it's the second(which one is you). For the conuiugation of "to be", you have only to find the really subject of the phrase. In this case it was "which one", so the third person.