It turns out: -더니

At its heart, 더니 connects two facts based on your personal experience. Think of it as a bridge between "Past Observation" and "Current Reality." You cannot use this for things you just heard about. You must have seen, felt, or heard the first part yourself. It is almost like saying, "Based on what I saw earlier, this happened." It often implies a cause-and-effect relationship. However, it can also show a simple contrast. For example, "It was cold yesterday, but it is warm today." The magic happens in the transition. You are telling the listener that you noticed the change. It is not just a random fact. It is a fact with a history. This grammar adds a layer of "I was there" to your speech. It is perfect for telling stories or explaining why things are the way they are now.

1. 1Using 더니 is actually quite simple once you get the hang of it. You do not need to worry about complex conjugations most of the time. Here is the breakdown:
2. 2Find the verb or adjective stem. This is the part before 다.
3. 3Attach 더니 directly to that stem. Do not add any tense markers to the first part.
4. 4Keep the first part in the present/base form even if it happened in the past.
5. 5Example: 자다 (to sleep) becomes 자더니.
6. 6Example: 춥다 (to be cold) becomes 춥더니.
7. 7Example: 공부하다 (to study) becomes 공부하더니.
8. 8Remember, the past tense is implied because 더니 itself carries the meaning of "past observation." You are looking back at something. So, you do not need to say 공부했더니 if you are describing someone else's behavior (that is a different grammar point entirely!). Just keep it simple. Stem plus 더니 does all the heavy lifting for you.

You use 더니 in three main scenarios. First, use it when an action leads to a result. If your brother ate five pizzas and now has a stomachache, 더니 is perfect. 피자를 많이 먹더니 배탈이 났어요. (He ate a lot of pizza, and now he has an upset stomach). Second, use it for contrast. If a coworker was super shy but is now the life of the party, use 더니. 처음에는 낯을 가리더니 이제는 말을 잘하네요. (He was shy at first, but now he speaks well). Third, use it for things that changed over time, like the weather or prices. It is great for those "It used to be like this, but now it's like that" moments. Think of it like a grammar traffic light changing from red to green. You saw the red, and now you see the green.

There are a few big "No-Nos" with 더니. The most important rule? Do not use it for yourself. In 99% of cases, the subject of the first part cannot be "I" (내가). You cannot "observe" yourself in the same way you observe others. If you want to talk about something you did, you should probably use 았/었더니. For example, 내가 공부하더니 sounds very strange to a Korean ear. It is like you are watching yourself from outside your own body. Also, do not use it for things you did not personally witness. If you heard from a friend that it rained, you cannot use 더니 to describe it. You have to be the primary witness. Finally, avoid using it for future predictions. This grammar is rooted in the past and the present. It is about what has already happened or is happening right now.

Yes, even native speakers mess this up sometimes! Well, not really, but learners definitely do. The number one mistake is the "Subject Trap." People often try to use 더니 with "I" as the subject. Stick to 3rd person subjects like friends, family, or the weather. Another common slip-up is adding 았/었 to the stem. While 았/었더니 exists, it has a different meaning and is used for 1st person subjects. If you are talking about someone else, just use the plain stem. Also, watch out for the second half of the sentence. It should usually be in the past or present tense. Trying to use a future tense at the end usually results in a clunky sentence. Think of it like trying to wear socks over your shoes. It just does not fit! Keep the focus on the change you actually saw.

You might be thinking, "Wait, how is this different from 았/었더니?" Great question! Think of 더니 as the "Observer" grammar and 았/었더니 as the "Participant" grammar. Use 았/었더니 when *you* did something and discovered a result. Use 더니 when you saw *someone else* do something. It is a subtle but huge difference. What about 니까? 니까 is a general reason or cause. 더니 is more specific because it requires that "past observation" element. 니까 can be used for things you didn't see, but 더니 cannot. Finally, compare it to 데. While 데 provides background info, 더니 emphasizes the change or result much more strongly. It is the difference between saying "It was raining, and..." versus "It was raining, and then suddenly the sun came out!"