-더니 / -았더니: 'I Noticed X, Then Y'

English hands you one all-purpose bridge for "A happened, and then B": and then, so, after that. Korean, because it builds these connectives on the retrospective -더-, splits that single English bridge into two — and the fault line runs along grammatical person. Bare -더니 reports something you witnessed about a second- or third-person subject and pairs it with a resulting change. -았더니 reports your own prior action and pairs it with a consequence you then discovered. Choosing between them is not a matter of taste; it is grammatically determined by who did the first action.

Bare -더니: "I saw A about someone, then B"

Attach -더니 to the first clause's verb stem. It says: I personally observed situation A (about someone or something other than me), and then B — a change, a result, or a contrast — followed, which I also noticed.

The subject of the -더니 clause is typically second or third person (아까는 웃더니… "you/she was laughing earlier, then…"). And because -더니 carries -더-, you must have witnessed clause A firsthand.

Notice the recurring shape: a change of state or a contrast between the two clauses. The sky was dark, and the change is that rain now falls; she was laughing, and the contrast is that she now cries. -더니 is at its most natural precisely when B is a turn or a development from A, observed by you.

-았더니: "I did A, then found B"

Now switch to your own action. When you are the doer of the first clause and the second clause is a result you then discovered, the ending is -았/었더니 — the past marker -았/었- fused onto -더니. Vowel harmony picks -았 or -었 exactly as elsewhere.

The meaning: I did A, and (turning my attention afterward) I found B to be the case. The first clause is almost always first person; the second is a state or outcome you registered as a consequence.

The direction is fixed: my action → the effect I discovered. 아침을 안 먹었더니 배가 고파요 flows "I skipped breakfast" therefore "I'm now hungry." You cannot reverse it into a plan ("I skipped breakfast, so I'll eat later") — the second clause must be a discovered result or state, not something you intend to do.

The two side by side

Because both are anchored in -더-, both keep its firsthand-witness restriction: neither can head a chain of pure hearsay. You cannot use -더니 to relay something you only read or were told — it always reports what you yourself saw or did.

Why this splits an English speaker

English lumps both under "and then / so," with no built-in signal for who did the first thing or whether you witnessed it. So learners reach for one of the two forms at random. The two errors that follow are mirror images: putting bare -더니 on their own action (which forces the odd reading "I watched myself do this"), or slapping -았더니 on someone else's action (which mis-assigns the deed to the speaker). Lock the rule in as a single question: whose action is clause A? If it's mine → -았더니. If it's someone/something I watched → bare -더니.

Common Mistakes

1. Bare -더니 with your own action. For a result flowing from what you did, use -았더니.

2. -았더니 for someone else's action. -았더니 assigns the first action to the speaker; a witnessed third-person sequence wants bare -더니.

3. Putting a plan or intention in the second clause of -았더니. Clause B must be a discovered result/state, not something you intend to do.

4. Using -더니 to relay hearsay. -더니 reports what you witnessed; it cannot head a chain built on news or others' reports.

Key Takeaways

• Both connectives sit on the retrospective -더-, so both report firsthand experience — never hearsay.
• Bare -더니: you witnessed A (2nd/3rd-person subject) → B, a change or contrast you noticed. 웃더니 우네요.
• -았/었더니: you did A (1st person) → B, a result you discovered. 걸었더니 다리가 아파요.
• The split is decided by who does clause A, not by mood or politeness.
• The direction of -았더니 is fixed: my action → discovered effect; clause B is a result/state, never a plan.