-고서야

A-고서야 B: B happens (or becomes possible) only after A is done, often describing a late realization.

시간·순서Time & sequence

Han: The verb before -고서야 takes no tense ending; past tense is marked only at the end of the sentence (알았어요).

Examples

그 사람을 직접 만나 보고서야 오해였다는 것을 알았어요.Only after actually meeting that person did I realize it had been a misunderstanding.

이렇게 매일 늦게 자고서야 어떻게 아침에 일찍 일어나겠어요?If you go to bed this late every day, how can you get up early in the morning?

건강을 잃고서야 건강이 얼마나 소중한지 알게 됐어요.Only after losing my health did I realize how precious it is.

엄마가 되고서야 부모님의 마음을 이해할 수 있었어요.Not until I became a mother could I understand how my parents felt.

V+고서야

듣다 → 듣고서야, 잃다 → 잃고서야

Do not put past tense on the verb before -고서야. Tense is shown at the end of the sentence: 듣고서야 알았어요, not 들었고서야.
It attaches to action verbs, usually with the same subject in both clauses. With points in time, -아/어서야 is more natural (10시가 되어서야).
The second clause cannot be a command or a suggestion (-세요, -(으)ㅂ시다).
Followed by 어떻게 ... -겠어요?, it becomes rhetorical: 'if you do A, how could B possibly happen?'

Wrong: ~~집에 도착했고서야 지갑이 없다는 걸 알았어요.~~

Right: 집에 도착하고서야 지갑이 없다는 걸 알았어요.

The verb before -고서야 takes no tense ending; past tense is marked only at the end of the sentence (알았어요).

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