in order to add some more logic and structure: 1) when speaking about time you should use -에 this works every time, no additional rules :) ex) 3시에 친구를 만날 거예요 = I(we,he,she,they) will meet with friends at 3 o'clock. 2) when speaking about location of somebody or something (but NOT of location of an action!) you should use -에 with -에 here you only say that something or somebody IS somewhere. for somebody is DOING something somewhere or something BEING HAPPENING somewhere --> check #3 ex) Where are you? - 저는 학교에 있어요. = I am at school. Where is the grandfather? - 할아버지께서는 수영장에 계세요. = Grandfather is in the swimhall. 3) when speaking about location of an action or activity you should use -에서 ex) Where do you study? - 저는 학교에서 공부해요. = I study at school. Where does grandfather swim? - 할아버지께서는 수영장에서 수영을 하세요. = Grandfather swims in the swimhall. 4) when speaking about direction -에 means 'to' and -에서 means 'from' ex) Where are you going to? - 저는 학교에 가요. = I am going to school. Where are you going from? - 저는 학교에서 가요. = I am going from school. Where are you from? - 저는 미국에서 왔어요. = I am from the USA. 5) regarding the position of -에 and -에서 these are POSTfixes, and are being placed only AFTER the word which they belongs to, and WITHOUT SPACE between the word and 에(서). so, if you see 에(서) before a word (like in your example, 에 was written before 마음), it simply means that 에(서) belongs to the previous word. ;) hope the scheme is more or less clear now :)

When it comes to movement, they are opposites: '에' means 'to', and '에서' means 'from'. 캐나다에 갔어요 - I went to Canada 프랑스에서 오셨어요 - He came from France 에 is used to say that someone or something is somewhere, while 에서 is used to say that something is the location of an action. 방에 있어요. I am in the room 학교에서 공부해요 – I study at school There are other usages too, but hopefully that's a start!

-에 is used for destination -에서 is used for origin -에 and -에서 can be used not only with physical place but also with conceptual place.

Thank you, for your answers! ^_^

um..It's difficult to explain the subtle difference, because they have various way to using. -에서 is used to express where happen any activity or action. For example, 나는 집에서 휴식을 취할 것이다. (I will rest in my home.) 직장에서 안 좋은 일이 있었니? (What is wrong in your workplace?) -에 is used to express that existence of something or where something occur. For example, 바다가 보이는 마을에 살다. (I live in village overlooking the sea.) 그는 뇌에 종양이 생겼다. (He developed a brain tumor.) And -에 is also used to express where to reach. For example, 나는 일본에 여행을 갔었다. (I traveled to Japan) but If you want to explain the experience traveling to japan, "나는 일본에서 여행을 한 적이 있다."<--this sentence is more proper. I hope that my answer helps you.