I wrote an entry on both (으)니까 and 더라, 았/었더라 which may be helpful to read before this. I believe if you understand both of these grammar points, you will almost automatically understand how the -더니 pattern works.
-더니 follows the same principles as -더라, but is used in the middle of a sentence instead of the end. It still allows the speaker to express something that they experienced, saw, observed, noticed directly to someone else. Please note for the purpose of brevity I will refer to this list of words as simply saw, and the exact underlying word will change depending on the context. The second part of -더니, the “니” is similar to the -(으)니까 meaning. Based on the actions or state of affairs seen in the first clause, the speaker has become aware of or is made to be aware of the actions or state of affairs in the second clause. This final definition is a bit wordy but very important!
Some sample sentences:
- 그 사람이 그렇게 능력을 인정받더니 결국 금방 승진했잖아. This sentence can be broken into two clauses, one before the 더니 and one after it. The first clause can be a standalone sentence if you add 라 at the end of it – “그 사람이 그렇게 능력을 인정받더라”, which would translate to “I saw that he was getting acknowledged for his ability”. The 니 particle expressed the meaning of “becoming aware of” or “to be made aware of” following from the actions in the first clause. When both clauses are combined it translates to “I saw that he was getting acknowledged for his ability and (because of this)/(after seeing this) he was promoted”.
- 어제는 비가 오더니 오늘은 눈이 오네요. I saw that it was raining yesterday and now today it’s raining.
- 그 여자가 아이한테 화를내더니 아이의 손을 때렸어. I saw that she was getting angry at her son and then she hit his hand
- 수지는 어렸을때 예쁘더니 이제는 별로네. I saw that Suzy was pretty when she was young, and now she’s not that great.
- 제 컴퓨터가 느려지더니, 이제 안 켜져요. I saw that my computer had slowed down, and now it doesn’t turn on.
There are some rules to this grammar I have purposely left after the sample sentences.
- This pattern is generally not used in the first person. My interpretation of why is because it is strange to say “I saw that I had done something, and then now I’ve become aware of something else”. Don’t take this literally, as it is possible to say in English “I saw what I had done and immediately regretted it”. This would be better translated as “내가 했던 걸 보니까 바로 후회됐다”.
There is a variation of this in the “past tense” construction 았/었더니 you can read about that here