In general you can use : 다른 쪽 (lit. another side) 반대 쪽 (lit. the other side [symmetrically]) If you want to say 'from the other side of the mirror', it is more right to say : 거울 뒤쪽에서 (lit. from the side behind the mirror) I think you can also say 건너편. 건너편 literally means 'the opposite side' of something that you can cross (e.g. a road, a street, ...). You can say 거울 건너편에서 (the opposite side of the mirror). Same for 'from the other side of the door', you should say 'from behind the door', 문 뒤쪽에서