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What rules make “Remember me, who am your friend” grammatical?
This is a line spoken by the Emperor to Luke in Star Wars. I always wondered if this is grammatically correct.
Luke says something like "You are mistaken ..." which the Emperor answers with
No, it is you who are mistaken!
Why wouldn't he say
No, it is you who is mistaken!
instead?
I don't know what rules apply here but my stomach tells me the latter is (at least also) correct, although you would say "you are mistaken". It feels as if he should be referring to "you" in the third person.
Could you please shed some light? Are both correct or --– if not --– which one is correct? And why?