"Which man is old?"
Translation:Który mężczyzna jest stary?
Why is it "jest stary" and not "jest starem"? I thought there was always instrumental case after jest unless preceded by "to".
Your knowledge is correct, but in this case "stary" meaning 'old' is and adjective (not a noun) and the adjective should take nominative (remember abot gender also).
He is old. = On jest stary.
She is old. = Ona jest stara.
It is old. = Ono jest stare.