I thought that "когда открыт магазин?" would mean the same as "когда открыли магазин?"
Originally Posted by it-ogo
It can be understood even that way but without the context "when the shop is open (usually)?" is the preferrable translation.
This is because we omit "был" in such questions. Full question should be: "Когда был открыт магазин?" and the answer: "Сегодня - в девять утра, а вообще-то в 1980 году"