Going places

On day 15, 에 marked where something is: 저기에 있어요. Here the same particle marks where something is heading. That is less odd than it looks, because 에 really marks a point in space and leaves the verb to say what happens there: with 있어요 you are at the point, with 가요 you are moving towards it. Unlike 은/는, 이/가 and 을/를, 에 has only one form, so there is nothing to choose.

English switches to 'come' when you move towards the person you are talking to: 'I'm coming!'. Korean does not. The verb follows your actual movement away from where you stand, so if you leave your spot to go to the listener, it is still 가요. A flatmate calling you to dinner hears 지금 가요, literally 'I'm going now', and that is exactly right. Reach for 와요 out of English habit and it sounds as if someone else is on the way.

와요 is movement towards the speaker's position and 가요 is movement away from it, and the speaker is always the fixed point. That is why a friend in Seoul says 한국에 와요 about your visit, while you, still at home in London, say 한국에 가요. The same pair sits inside two phrases from day 1: 안녕히 가세요 is said to the person going, and 안녕히 계세요 to the person staying put. Once you see where the speaker is standing, the choice makes itself.