Core Grammar Rule: ~더니
This pattern connects a past observation or action by the speaker to a resulting change or newly discovered fact.
Meaning & Usage
Meaning: It translates to "I saw/did X, and as a result, Y happened" or "I noticed X, and now Y is the case." The key is that the speaker directly experienced or observed the first clause (the cause).
When to use: Use it to explain a direct consequence you've observed. It's very common for describing changes in one's own body after doing something (e.g., "I exercised, and now I'm sore") or changes in someone else that you've witnessed (e.g., "He studied so hard, and now he's passed the exam"). The subject of the first and second clause is often different when observing others, but the same when talking about your own actions.
Conjugation Rules
Rule: Simply attach ~더니 to the stem of a verb or adjective, regardless of whether it ends in a vowel or a consonant. It's used with the past tense form of the verb stem for completed actions, but often the simple stem is used for ongoing states or actions leading to the result.
Verb: 먹다 (to eat) → 먹더니 (ate, and now...) / 운동하다 (to exercise) → 운동하더니 (exercised, and now...)
Adjective: 예쁘다 (to be pretty) → 예쁘더니 (was pretty, and now...)
**Verb Stem:** 가다 (to go) → 가더니 / **Adjective Stem:** 바쁘다 (to be busy) → 바쁘더니
Expressing Guesses and Regrets with ~(으)ㄹ걸(요)
This versatile ending allows you to express either an uncertain guess about a situation or a feeling of regret about a past action.
Expressing a Guess: ~(으)ㄹ걸요
Context: Use ~(으)ㄹ걸요 when you are making a soft, polite guess about something you aren't 100% sure about. It adds a nuance of "probably" or "I think..." and is less direct than other guessing patterns. It's often used when speculating about the reason for something, like in the dialogue: "미세먼지가 심해서 그럴걸요." (It's probably because the fine dust is bad.)
Example: 그 사람은 벌써 집에 갔을걸요. (That person has probably gone home already.)
Expressing Regret: ~(으)ㄹ걸 (그랬어요)
Context: Use ~(으)ㄹ걸 or ~(으)ㄹ걸 그랬어요 to say "I should have..." or "I wish I had...". It expresses regret over a past action that you didn't take or a choice you didn't make. The 그랬어요 part makes it more explicit and slightly more formal. Friends often just say ~(으)ㄹ걸.