[Korean grammar] -고서 Expressing Time and Sequential Behavior

가: 오늘 왜 안경을 안 썼어요?
Why didn’t you wear your glasses today?
나: 아침에 안경을 책상 위에 올려 놓고서 잊어버리고 그냥 나왔어요.
I placed my glasses on my desk this morning and then just left, leaving them behind.

가: 어제 수영을 하다가 발에 쥐가 나서 아주 힘들었어요.
Yesterday, I was swimming and got a cramp, and it really hurt.
나: 그러니까 운동을 할 때는 꼭 준비운동을 하고서 해야 돼요.
That’s why you have to do warm-up exercises (first) and then exercise.

This expression is used to indicate that the first action was completed before the subsequent action started. Moreover, there is a before-and-after relationship between the actions in the preceding and following clauses. This expression can only be used with verbs.

* 저는 아침마다 조깅을 하고서 학교에 옵니다.
Every morning, I go running (first) and then come to school.

* 책을 읽고서 친구들과 토론을 했어요.
I read a book and (after that) had a discussion with some friends.

 1. This expression cannot be used with the tense-indicating expressions -았/었- or -겠.
• 인사만 했고서 헤어졌어요. (X)
인사만 하겠고서 헤어지려고 해요. (X)
->인사만 하고서 헤어졌어요. (〇)

2. Both the phrases -고 and -고 나서 can be used to indicate the order of events.
• 책을 읽고서 감상문을 썼어요.
= 책을 읽고 감상문을 썼어요.
= 책을 읽고 나서 감상문을 썼어요.