-더니 vs -었더니: Watched It, or Did It Then Found

You watched the day

A kid has been outside since morning. Now the house is quiet.

하루 종일 뛰어다니더니 지금은 자요.
haru jongil ttwieodanideoni jigeumeun jayo — Ran around all day, and now is asleep.

더니 hangs a past situation you could see, then the sequel. Classroom drills keep that participant in both halves. The running is not “I ran, then I discovered sleep.”

Swap in the past-marked twin.

하루 종일 뛰어다녔더니 지금은 자요.

That line asks the other story: you did the running, then found the sleep. If you were only watching the kid, it is the wrong hinge.

You did it, then found it

The door was closed. You opened it.

문을 열었더니 고양이가 들어왔어요.
muneul yeoreotdeoni goyangiga deureowasseoyo — I opened it, and a cat came in.

었더니 is the past-marked chunk. Discovery stories live here: 가봤더니 휴일이었어요. 검색해 봤더니 후기가 좋았어요. You acted. Then the world answered.

문을 열더니 고양이가 들어왔어요 puts the opening in the watched-situation seat. If your hands turned the handle, classroom Korean wants 열었더니. Do not borrow 더니 because the letters look cheaper.

Weather makes the same cut. 날씨가 춥더니 눈이 왔어요 watches the cold, then the snow. 창문을 열었더니 추웠어요 is your act, then the cold you met. 열어서 들어오세요 is a different error: -아서 cannot sit in front of a command. That job is -(으)니까, and it is not this page.

Same medicine, two seats

약을 먹더니 좀 괜찮아졌어요 — you watched them take it, then they looked better.

약을 먹었더니 머리가 덜 아파요 — you took it, then you felt the change.

Do not leave the page thinking both are fine if you add 요. One form watches a past scene onward. The other marks your act, then the find. Pick the story first. The letters 더니 will try to fool you. They are not a style pair. If you only remember one failed line, remember 열더니 said with your own hand still on the door. That is the seat that belongs to 열었더니.