-았/었더니 — and, so

This expression is used to indicate that something happened as a result of something the speaker did or said previously.

Details

1. This expression can also be used to indicate the speaker's discovery of what is expressed in the following clause after doing the action stated in the preceding clause. In this case, the expression -(으)니까 can also be used to indicate the same meaning.

백화점에 갔더니 사람이 많았어요.

그분을 만나 봤더니 아주 친절한 분이셨어요.

2. When an action is described in the preceding clause, the person who performs the action must be '나' that is, the 1st person.

(내가) 오래간만에 운동을 했더니 기분이 상쾌해요.

(내가) 1년 동안 한국에 살았더니 이제 한국 생활에 익숙해요.

However, when the preceding clause is relating what someone else said, the subject of the clause can be in the 3rd person.

3. There are cases when the subject of the preceding clause is in the 3rd person. In such cases, the speaker is recollecting the, completed action or behavior of someone else. In addition, the subjects of the preceding and following clauses are different, with the following clause typically describing a reaction to the action or behavior described in the preceding clause.

아키라 씨가 피아노를 쳤더니 사람들이 박수를 쳤습니다.

Akira played the piano, so (in response) the people (crowd) applauded.

사람들이 웃었더니 게이코 씨 얼굴이 빨개졌어요.

Everyone laughed, so (as a result) Keiko's face turned red.

정호 씨가 늦겠다고 했더니 수진 씨가 화를 냈어요.

Jeongho said he'd be late, so (upon
hearing that) Sujin got angry.

While -더니 and -았/었더니 are similar in meaning, they are different in the following respects.

Kimchi Reader

Used to narrate a sequence of events observed by the speaker. There is often a nuance that the first clause becomes the cause of the second clause. The subjects of both clauses must be different. Note that the speaker's past self and present self can be considered different subjects, and so first-person narration is possible.