더니 ① — but

Used when the speaker wants to refer to a change compared to the past that they have directly seen or experienced. The speaker themself cannot be the subject.

Details

Possible alternative form -더니만.

네가 예전엔 이런 음식을 안 먹더니만 이제는 잘 먹는구나.

You didn't eat this kind of food before, but now you eat it well.

Can be used with nouns if they are followed by 이다.

Korean Grammar in Use

This expression is also used to indicate a contrast between what one experienced or knew to be the case in the past and the current state of affairs.

Details

2. This expression can only be used when the subject is in the 2nd or 3rd person, not the 1st person.

열심히 공부하더니 1등을 했어요. ✕

However, it is possible to use -더니 with a 1st person subject when the speaker refers to himself or herself in an objective manner.

(내가) 며칠 전부터 피곤하더니 오늘은 열도 나고 아파요.

I haven't felt good for the last few days, and now I have come down with a fever.

(내가) 젊었을 때는 사람들 어름을 잘 기억하더니 요즘은 통 기억을 못하겠어요.

I remembered people's names very well when I was young, but now I can't remember them.

3. Both the subject and topic of discussion of the preceding and following clauses -더니 must be the same.

동수 씨가 노래를 하더니 사람들이 박수를 쳤어요. ✕

The subjects of the preceding and following clauses are different, and thus the sentence is grammatically incorrect.

4. The future tense cannot be used in following clauses this expression.

라라 씨는 꾸준히 연습하더니 세계 최고의 선수가될 거예요. ✕

5. This expression is used when recalling an action or state of affairs directly witnessed in the past, with the succeeding action or state of affairs occurring directly following the first.

소연이는 남자 친구한테 화를 내더니 밖으로 나가 버렸어요.

Soyeon got mad at her boyfriend and then (soon after) went outside.

동생은 집에 들어오더니 갑자기 울기 시작했어요.

My little brother/sister came into the house and then suddenly started crying.

When used to indicate contrast, -더니 can be used in place of -(으)ㄴ/는데, but note the following differences.