You are right. It is an exception; drop '어' when the verb root already ends with '-ㅕ'. 켜다-켜어요-켜요 (turn on) 펴다/펴요 (unfold, spread) ***Note: When the verb/adjective predicate ends with 'l' and add '-어요', the contraction happens such as: 마시다-마시어요-마셔요 (drink): 커피를 마셔요. 치다 - 치어요 - 쳐요 (hit): 공(ball)을 쳐요. 꽃이 피다 - 꽃이 피어요 - 꽃(flower)이 펴요 (bloom) 시다 - 시어요- 셔요(is sour): 김치가 셔요.

First of all, turn on as in turn on the light is 켜다 in Korean (not 겨다) If that's what you meant, another way of saying this would be 켜요 (as a way of telling others to turn on the light). E.g) 불 좀 켜요 불 좀 켜 주세요 방이 어두우니까 불 좀 켜주세요. If you could make your question more clear, then i would be able to help you out!