V-더니 is used to describe an observation of the speaker about people or things, or a change that came to be known through personal experience, and always precedes a statement resulting from that observation. A good example in English/Korean of how this pattern would be used: "my friend used to eat a lot of ice cream before더니 these days he even hates the sight of ice cream." V-더니 can therefore only be used in the middle of a sentence and can never end a sentence.
The clause before V-더니 can only be in the second or third person, so this pattern is therefore not appropriate to describe things the speaker has done. It is used to describe what the speaker has noticed about the external environment. Warning: while V-더니 cannot be used in the first person, V-았/었더니 can, and will be the object of a different lesson.
Other points about the pattern V-더니:
- the subjects in the clause preceding V-더니 and after it must be the same.
- the clause after V-더니 must be logically connected to the observation preceding it; i.e. both should be about the same topic.
- Korean sentences with V-더니 are challenging to translate in English without adding more context to the translations. If you look at the examples below, V-더니 can be translated into different ways to fit the original sentence better.
(I noticed before that) My friend ate a lot of ice cream in the past but now he hates even the sight of ice cream.
(I noticed) there were a lot of clouds in the morning, but they have all disappeared in the afternoon.
Now that my friend has started working, we cannot meet a lot because he is busy (implies that the speaker and the friend used to meet a lot before).
When you were young you couldn't go to bars but I see that you can now!
(I noticed that) You didn't study and so I see you failed your test!
(I noticed that) Because My friend saved a lot of money, he became rich.
(I noticed that) My friend has changed a lot after he fell in love.