In English we have separate words for coming and going but their distinction is not as strict as when these words are used in Korean. Since I’ll be leaving for Korea in a few months I can safely say in English “When I go to Korea”, or I can say “When I come to Korea” without any major changes in meaning.
However, in Korean I always get this confused when I talk to my language partner. He will often present me with a new idea for things to do when I arrive like places to eat, sights to see, people to meet etc. Therefore he often uses sentences with 서울에 오면 and 한국에 올 날 meaning “when you come to Seoul” and “the day you come to Korea” respectively. Since these are the words he’s feeding me, I frequently make the mistake of repeating them in my own sentences, of course always resulting in me being corrected instantly. Since he’s already in Seoul it’s perfectly fine for him to use the word come (오다), but I can’t use this word since I’m not there yet. I have to use the word go (가다) as in 서울에 가면 and 한국에 갈 날, “when I go to Seoul” and “the day I go to Korea”. Like so many words in Korean, it depends on who you are, where you are, and who you are talking to.
In fact, these two verbs play a very important and quite interesting role in the Korean language. They often attach to other verbs in order to convey different meanings and nuances. Take this sentence for instance:
아주 오랜 시간 동안 좋아해 온 사람 (A person I have liked for a very long time)
Here the conjugated verb 오다 (come) is attached to the verb 좋아하다 (like) indicating that the act of liking someone has been going on for a while and continues at least until the present moment. Another sentence (this is actually taken from my Korean speech, which I delivered at the speech contest in October) using 가다 (go) could be:
한국 문화를 알아가면서 한국을 더 사랑하게 되었습니다 (As I learned about the Korean culture, I fell in love with Korea)
English doesn’t really convey the same nuance of continuity in the latter example. This sentence uses 가다 to emphasize that learning about the Korean culture is still very much work in progress.
Finally, when these two verbs attach to the verb 가지다 (to have) as in 가져오다 and 가져가다 they mean “to bring” and “to take” respectively.
There’s also another fixed Korean expression, 오락가락, which may be used about things that are unstable. It roughly translates as “coming and going” or “on and off”. My very first language partner once told me during one of our first meetings that the weather here seemed to be very 오락가락.
The sun was shining brightly when I got up this morning, and yet I still ended up biking to work in pouring rain, only to find that the sun had come out again when I had had my first cup of coffee. 오락가락 indeed!