In short

The retrospective -더- reports something you personally saw, heard, or felt in the past and are now recalling. In speech that is -더라고요 (“I noticed that…”) with its casual twin -더라; as a noun-modifier it is -던 (habitual or left unfinished) and -았/었던 (completed and now over). 그 카페 커피가 맛있더라고요 (masitdeoragoyo, the coffee there was good, I found).

The rule

-더- is Korean’s firsthand-evidence marker: it means the information came in through your own eyes, ears, or feelings on some past occasion. It never takes 으 and never splits by vowel harmony - it just clips straight onto any stem.

Step 1 - -더라고요: reporting what you witnessed

Verb/Adjective stem + -더라고요. It presents a fact as a fresh discovery you made: 어제 가 보니까 사람이 많더라고요 (manteoragoyo, there were a lot of people, I found). Stack the past -았/었더라고요 when the event was already finished when you noticed it: 벌써 갔더라고요 (gatdeoragoyo, it/he had already gone, I saw). For a noun it is -이더라고요: 학생이더라고요 (haksaengideoragoyo, turned out to be a student).

Step 2 - the person rule (this is the tricky part)

Because -더- reports what you observed from the outside, it clashes with your own actions - you can’t watch yourself. So report your own actions with the plain past instead. But it flips for feelings: your own inner reactions are exactly what -더- is for, while someone else’s feelings you can only infer, so those need -어하다.

• someone else’s action you saw -> 친구가 공부하더라고요 (chinguga gongbuhadeoragoyo, my friend was studying, I saw).
• your own action -> plain past: 저는 공부했어요 (jeoneun gongbuhaesseoyo), not 저는 공부하더라고요.
• your own feeling -> 저는 무섭더라고요 (jeoneun museopdeoragoyo, I found it scary).
• someone else’s feeling -> 친구는 무서워하더라고요 (chinguneun museowohadeoragoyo, my friend was scared).

Step 3 - the casual -더라 and the background -던데요

-더라 is the 반말 form of -더라고요, for friends: 그 영화 진짜 재미있더라 (jaemiitdeora, that movie was really fun). -던데요 (from -더- + -는데요, lesson 41) lays a witnessed fact down as background that invites a reaction: 아까 보니까 문이 열려 있던데요 (yeollyeo itdeondeyo, the door was open when I looked, you know). The past -았/었- stacks on -더라 and -던데요 exactly as it does on -더라고요 - 왔더라 (watdeora), 컸던데요 (keotdeondeyo) - and, unlike -더니, it changes nothing but the completeness of what you saw.

Step 4 - -더니 vs -았/었더니

-더니 links an earlier situation you watched to a change or result, and its subject is 2nd or 3rd person: 어제 아프더니 오늘은 괜찮네요 (eoje apeudeoni oneureun gwaenchanneyo, you/he were sick yesterday, and today you’re fine). Swap in the past -았/었더니 and it becomes first person: you did something, and then something followed - 아침을 안 먹었더니 배고파요 (achimeul an meogeotdeoni baegopayo, I skipped breakfast, so now I’m hungry).

Step 5 - the modifier forms -던, -았/었던, and plain -(으)ㄴ

As a noun-modifier, -더- becomes -던 and marks the past as recalled - either habitual (“used to”) or unfinished/interrupted: 다니던 학교 (danideon hakgyo, the school I used to attend), 마시던 커피 (masideon keopi, the coffee I was drinking). -았/었던 adds completion and discontinuity - it happened, and it’s now over or no longer true: 갔던 곳 (gatdeon got, the place I once went), 예뻤던 얼굴 (yeppeotdeon eolgul, the face that used to be pretty). Plain -(으)ㄴ (lesson 24) is the neutral completed past with none of that recall/discontinuity flavour: 먹은 빵 (meogeun ppang, the eaten bread).

Forms & patterns

The spoken -더- endings (they attach straight onto any stem - no 으, no vowel-harmony split):

The modifier forms (-던 attaches directly; -았/었던 needs the past, picked by vowel harmony):

-더니 vs -았/었더니 - the person and tense pivot:

Person and evidence at a glance (-더- always encodes firsthand past evidence):

Examples

Common mistakes

• 저는 매일 학교에 가더라고요. (Jeoneun maeil hakgyoe gadeoragoyo.) -> 저는 매일 학교에 갔어요. (Jeoneun maeil hakgyoe gasseoyo.) - -더- reports what you watch from the outside, so it clashes with your own action; report your own actions with the plain past -았/었-.
• 친구는 그 영화가 무섭더라고요. (Chinguneun geu yeonghwaga museopdeoragoyo.) -> 친구는 그 영화를 무서워하더라고요. (Chinguneun geu yeonghwareul museowohadeoragoyo.) - a bare feeling-adjective + -더- can only describe your feeling; for someone else’s feeling use -어하다 (as written, 친구는 무섭더라고요 would mean “my friend was scary, I found”).
• 저는 아침을 안 먹더니 배고파요. (Jeoneun achimeul an meokdeoni baegopayo.) -> 저는 아침을 안 먹었더니 배고파요. (Jeoneun achimeul an meogeotdeoni baegopayo.) - when your own action causes the result, use -았/었더니 (past); plain -더니 needs a 2nd/3rd-person situation you witnessed.
• 책상 위에 마신 커피가 있어요. (Chaeksang wie masin keopiga isseoyo.) -> 책상 위에 마시던 커피가 있어요. (Chaeksang wie masideon keopiga isseoyo.) - 마신 = drank (finished, gone); coffee you left half-drunk is 마시던 (unfinished). -(으)ㄴ seals the action; -던 leaves it open.
• 그 집 진짜 좋더라구요. (Geu jip jinjja joteoraguyo.) -> 그 집 진짜 좋더라고요. (Geu jip jinjja joteoragoyo.) - the standard spelling is -더라고요; -더라구요 is a colloquial mis-spelling of the same ending.

Tips

• -더- means “I was there.” Everything built on it - -더라고요, -더라, -던데요, -더니, and the modifier -던 - carries firsthand past evidence: you saw, heard, or felt it yourself. That is what sets it apart from the second-hand hearsay -대요 (lesson 59).
• Own action -> no -더-; own feeling -> yes -더-. You can’t watch yourself act, but you can feel your own feelings, so 저는 무섭더라고요 is fine while 저는 가더라고요 is not.
• -던 leaves it open, -았/었던 seals it. Habitual or unfinished = -던 (마시던 커피 is still on the desk); done-and-over = -았/었던 (마셨던 커피 is long gone).
• -더니 pivots on 았/었. No past marker = someone you watched changed or reacted; add 았/었 with a 저/제 subject = your own action produced the result.