The usage of 에/에서.

1. 에 is toward, you are heading somewhere. 에서 is from, you are from somewhere. Let's say if you want to say father went to the room. Father's heading somewhere so you use to, which is 에. So you say, 아버지가 방'에' 들어갔습니다. If you want to utilize 에서, then you should make a sentence like, Father came out from the room. Then it would be like: 아버지가 방'에서' 나왔습니다. 2. Basically, when you are at somewhere then you say '에서' since it means when something is inside. So everything with "someone doing something" should be with 에서. So it is always used as ~에서+verb. like, I play piano in my room -> 나는 방'에서' 피아노를 친다. or I study at home. 나는 집'에서' 공부한다. But when you want to describe someone or something is located at somewhere, but not particularly doing something. Like My mom in the hospital(My mom is in the hospital). -> 병원'에' 있는 우리 엄마(우리엄마는 병원'에' 있다). So in this sentence, 있다 is used as 'located at' or 'being somewhere'. And as I said if you want to use 에 as this meaning(not as 'toward') then it's always used as ~에 있다.