The plain workhorse for stringing things together: does this and that, either as a list or in sequence (끓여주고 "boil it, and...", 매니저 오빠랑도 친하고 "and I'm close with our manager too"). What foreigners notice is that speakers leave it hanging at the end of a sentence — 그래서 그렇게 해두고... — where it signals that more was going on but the speaker is letting it trail off, softening the statement. It also forms 지 말고 don't do it and instead (잊지 말고 "don't forget"). Do not confuse it with the 고 in 고 있다 (in progress) or in quoting, which are separate patterns built on the same syllable.