"The dog returns the ball."
Translation:개가 공을 돌려줍니다.
돌려져요 would be the conjugation of 돌려지다, but the word return is 돌려주다 so it would be 돌려줘요. however duolingo doesn't accept that either. Cause it's still in beta I guess sometimes it won't accept other politeness forms/other correct conjugations.