-더니

-더니 links something the speaker saw in the past with what happened next — a change, a result or a contrast.

감정·희망Feelings, hopes & regrets

Han: For your own action followed by a result, use -았/었더니.

Examples

민수 씨가 아까부터 기침을 하더니 결국 조퇴했어요.Minsu had been coughing for a while and in the end left early.

동생이 열심히 공부하더니 장학금을 받았어요.My younger sibling studied hard and (as a result) got a scholarship.

아침에는 맑더니 오후에 비가 왔어요.It was clear in the morning, but it rained in the afternoon.

V/A+더니

춥다 → 춥더니, 공부하다 → 공부하더니

The first clause is usually about someone else (not 'I'), something the speaker observed.
-았/었더니 is different: 'I did A and then found B' (first person).

Wrong: ~~제가 열심히 공부하더니 합격했어요.~~

Right: 제가 열심히 공부했더니 합격했어요.

For your own action followed by a result, use -았/었더니.

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