-아/어서 and -고 나서 has a difference even though both describe the first part of a two-part event. * -아/어서 is used when the two events are inseparably connected, with the two verbs having the same subject. (If multiple subjects are involved, -아/어서 falls back to the meaning of a reason or background (e.g. 선생님이 화를 내셔서 우리는 어쩔 줄 몰랐다 states cause and effect rather than sequential events). 서 can be dropped to make the transition between the two parts even smoother. - 짐을 싸(서) 탁자에 올려 놓았다. - 우리는 낚시해(서) 잡은 고기를 끓여(서) 먹었다. * -고 나서 makes the two events more distinct and separated, and the two parts can have their own subjects. It can be shortened to -고서, which then weakens the sense of separation somewhat. So -아/어서 shows a very tight connection, -고서 a more loose one between distinct events, and -고 나서 a case where the first part is fully completed before the second part starts. (-고서 can also be shortened to -고, especially when the sequence aspect is not so important, in which case it becomes almost indistinguishable from -고 as an enumerating "and"). - 숙제를 하고 나서/ 하고서 / 하고 친구한테 전화를 했다. (숙제를 해서 won't work because the actions are not directly related.) - 비가 오고 나서/오고서 날씨가 추워졌다. (different subject) ('비가 와서' is grammatically okay, but it sounds like a reason for the main clause, not two connected events.) Here's an example of how they can subtly vary in connotation. - 해가 지고 나서 달이 떴다. (clear separation in time) - 해가 지고서 달이 떴다. (similar, but less clear distinction) - 해가 지고 달이 떴다. (sequence is not emphasized - it starts to sound like two unrelated events mentioned together) All three are natural and reflect how the speaker views the events, but 해가 져서 달이 떴다 is not, because the two things are neither a closely connect sequential events nor a case of one causing the other.