What is the main difference between 에 and 에서?

There is some difference between 에 and 에서. It is very easy to find the difference.

When you use (at, in, on, to) in English to indicate a noun(place) 에 is used somehow related to that place. 에 is used to show motion towards a place, and the location of things including time.

Difference between 에 and 에서

에 is used if the noun is the destination of the action.
집에 갈래. Do you want to go home?
학교에 가요. I’m going to school.

에 is used to indicate a noun (date/time) on/at. Examples:
7시에 만나요. (Meet me at 7 o’clock.)
토요일에 얘기하자. (Let’s talk about it on Sunday.)

You can not use 에 like 지금에, 오늘에, 어제에, 내일에. These words are just used like this 지금, 오늘, 어제, 내일.
에서 is used to indicate a noun (place) where an action takes place.
여기서 기다려. (Wait here.) 여기서 is a short form of 여기에서.
아침에는 집에서공부하고 낮에는 학교에서 공부합니다.

에서 is used to show motion origination (from a place) place, and location of actions 에서 is used to indicate a noun (place) an action starts from.

어디서? – short form of 어디에서
도서관에서 오다 – To come from the library.
집에서 했다. I did it at home.
학교에서 가는 거야? will you be going there from your school?

에 is used when you want to say “at some location” with the 있다 or some variant verb
에서 is used when you want to say “at some location” with an action verb.
집에 있다. Stay at home
집에서 밥 먹다. Have lunch at home.

집에있다 I am at home (to be at home).
집에서 공부한다. I study at home. (to study at home)

에 is used to describe someone or something is somewhere. 에서 is used to describe something is the location of an action.
에 Examples:
학교에 가다 Go to school.
저는 미술관에 가요 I go to the museum.
우리 집에 에어콘 있어요 I have an air conditioner at my house.
병원에 갔어요. I went to the hospital.

에서 Examples:
학교에서 공부하다. Study at school.
학교에서 공부해요 I study at school
호텔에서 점심을 먹었다. I ate lunch at the hotel.
극장에서 왔어요 I came from the theatre.
극장에서 영화를 봤어요 I saw a movie at the theatre

‘에’ has the meaning of ‘to’, and ‘에서’ has the meaning of ‘from’ when it comes to movement.
저는 공부하러 프랑스에 갔어요. I went to France to study.
우리 친구는 캐나다에서 오셨어요. My friend came from Canada.

에 is used for destination
병원에 가기 싫어요. (I don’t want to go to the hospital.)

에서 is used to express for the origin of any activity or action
저는 서울에서 왔어요. (I’m from Seoul.)