-더니 — and

This expression is used when the speaker wants to refer to something directly seen or experienced in the past when describing how that thing has subsequently changed.

The seen or experienced content is stated as the reason or cause for the changed state of affairs being presently witnessed.

The form -더니만 is an alternate form of this expression.

Details

1. 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.

오후에는 덥더니 저녁이 되니까 쌀쌀하네요.

It was hot in the afternoon, but now it's chilly in the evening

마크 씨가 월초에는 많이 바쁘더니 요증은 좀 한가해진 모양이에요.

Mark was so busy at the beginning of the month, but now he seems a bit more free,

딸아이가 작년에는 여행을 많이 다니더니만 요즘은 통 밖에 나가지를 않아요.

(My) daughter did a lot of traveling fast year, but these days she doesn't leave the house at all.

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.

Kimchi Reader

Used to narrate a sequence of events directly observed by the speaker. The subjects of both clauses are usually but not always the same, and the speaker cannot be the subject. There is often a nuance that the first clause becomes the cause of the second clause.

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.