Beautiful : 아름답다. 예쁘다.
1. To a person :
(당신은) 아름다우세요 -> this is very polite and formal manner.
(당신은) 예뻐요 -> this is more informal so you can use this phrase to girl, girl-friend, sometimes to kids.
2. When I look at a beautiful picture/photo or a beautiful view.
아름다운 그림(사진)이네요. (this is beautiful picture/photo)
경치가 아름다워요 : this is beautiful scenery