You can say: - 한국어 강좌에 학생이 15 명 있어요 (There are 15 students in the Korean class) - 제가 다니는 한국어 강좌에 학생이 15 명 있어요 (There are 15 students in the Korean class that I'm attending) Your sentence is correct except for 에서. The reason is because 에서 is only for situations where something originates from the place mentioned. That is, you start the activity there, and finish either there or somewhere else. (like, in, at, or from) 에 is used if some activities which started somewhere else continues or finishes there. (like to, at, or in) 있어요 means "exist", but here it is more like "take the class" or "be enrolled in". Since people go to the class, and don't start their life or even the school work from that class, 에 should be used. Examples: - 나는 매일 학교에 가. I go to school everyday (I go there from a place external to the school) - 서울에서 이 사진을 찍었어: I took this picture in Seoul. (the activity of taking the picture started in Seoul) Sometimes the distinction is not that clear so both particles may be used. - 나는 부산에 살아 (I live in Busan) == 나는 부산에서 살아. (both are correct) - 우리 삶에 제일 중요한 것 (The most important thing in our lives) == 우리 삶에서 제일 중요한 것. (slight nuance difference)