~더니 (~deo-ni) – I (personally) experienced that someone did something before (and therefore/but) that happened (to that same someone)
This looks quite similar to [Grammar] ~았/었더니 but they are in fact very different. It’s extremely confusing at first but once you lay out the differences, it will be much clearer.
And no, the difference does NOT lie in the fact that ~더니 is present tense and ~았/었더니 is past tense.
Both grammar particles are used to describe a past situation/fact and the result that the speaker/writer personally experienced.
The following are 3 important properties to know about ~더니 (very different from ~았/었더니’s properties):
1. Can be used to express a contrast
2. Commonly used with second and third person subjects in the first clause
3. Both clauses refer to the same subject
친구는 피자를 사러 가더니 1시간 되었도 아직 돌아오지 않네.
(I personally saw that) My friend went to buy pizza but it’s been an hour and he’s still not back yet.
이른 아침에 엄마가 김밥을 만들더니 이제 어디 있나요?
(I personally experienced that) My Mom made Kimbaps early in the morning but where is it right now?
선생님은 분명히 이 제목을 가르쳐 주시더니 지금 또 가르치고 있네요.
(I personally experienced that) Teacher clearly taught this topic before but she’s teaching it right now again.
우리 강아지는 아까 먹더니 이제 자고 있어요.
(I personally experienced that) My puppy ate just now and (therefore) it is sleeping now.