는다고 하다

they say that; it's said

Passes on what somebody else said. Paired with 더라고요 it is clearly something you picked up secondhand: 배에서 밤에 항상 저 빨간 불을 켜놓는다고 하더라고요 'they said they always leave that red light on at night.' It also holds someone to their earlier words — 프로님 화내지 않는다고 하지 않으셨나요? And it is how you state a fact about the language itself: 한국에서는 약을 마신다고 하지 않고 먹는다고 해요.