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eeokeoneul kyeossneunde gapjagi kkeojyeosseoyo.

에어컨을 켰는데 갑자기 꺼졌어요.eeokeoneul kyeossneunde gapjagi kkeojyeosseoyo.
I turned on the air conditioner, but it suddenly turned off.
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Questions & Answers about eeokeoneul kyeossneunde gapjagi kkeojyeosseoyo.

Why is 에어컨을 marked with -을? Could it be 에어컨이 instead?

-을/를 marks the direct object of an action verb. Here, 켜다 (to turn on) takes an object: 에어컨을 켜다 = turn on the air conditioner.
If you used 에어컨이, it would usually make 에어컨 the grammatical subject, which fits better with sentences describing what happened to it (especially with intransitive verbs), e.g. 에어컨이 꺼졌어요 = the air conditioner turned off.
In this sentence, the first clause is about what the speaker did (turned it on), so 에어컨을 is natural.

What does -았/었는데 do in 켰는데?

-았/었는데 connects two clauses and often implies a contrast or an unexpected follow-up, like (I did X,) but/and then (Y happened).
So 에어컨을 켰는데 갑자기 꺼졌어요 suggests: the speaker turned it on, and unexpectedly it turned off right after / as a result of the situation.

Does -는데 always mean but?

No. -는데 can do a few jobs depending on context and intonation:

  • Background/setup: (I turned it on,) and then…
  • Contrast: (I turned it on,) but…
  • Mild explanation/lead-in: (It’s that I turned it on,) so… In this sentence, it’s commonly understood as and then / but then because the second clause is surprising.
Why is 켰는데 past tense if the action happened just now?

Korean often uses past tense to show a completed action that occurred before the next event, even if it was moments ago.
켰는데 = (I) turned it on (and then...) → that action is already complete before the turning off happens.

What’s the difference between 꺼졌어요 and 껐어요?

They come from different verbs:

  • 꺼지다 (intransitive): to go off / to turn off (by itself) → 꺼졌어요 = it went off.
  • 끄다 (transitive): to turn off (something intentionally) → 껐어요 = I turned it off. This sentence uses 꺼졌어요 to show the air conditioner turned off unexpectedly / not by the speaker’s deliberate action.
Why is the subject not stated? Who/what turned off?

Korean frequently omits subjects when they’re obvious from context. Here, 꺼졌어요 strongly implies the air conditioner went off (because it was just mentioned, and 꺼지다 describes something switching off).
If you wanted to make it explicit, you could say: 에어컨이 갑자기 꺼졌어요.

Where does 갑자기 go in the sentence? Can it move?

갑자기 (suddenly) is an adverb and is fairly flexible:

  • 에어컨을 켰는데 갑자기 꺼졌어요 (very natural)
  • 에어컨을 켰는데 꺼졌어요, 갑자기 (possible but more afterthought-like)
  • 갑자기 에어컨이 꺼졌어요 (also natural, focuses on suddenness earlier) Its position affects emphasis slightly, but all can be grammatically fine.
Is 에어컨을 켰는데 implying cause, like turning it on made it turn off?

It can imply that sequence and a mild sense of unexpected result, but it doesn’t strictly claim causation. It mainly says: 1) (I) turned on the AC,
2) then it suddenly turned off.
If you want clearer causation, you might add phrasing like 켜자마자 (as soon as I turned it on) or use other constructions depending on meaning.

Could I replace -는데 with -더니 here?

Often yes, with a nuance change:

  • 켰는데 꺼졌어요: neutral sequence + contrast/unexpectedness.
  • 켰더니 꺼졌어요: more like I turned it on, and (as a result) it turned off, often sounding like a discovered outcome.
    Both can work, but -더니 tends to feel more like observation/result from the speaker’s experience.
What politeness level is 꺼졌어요?

-어요/아요 is the standard polite conversational style.

  • Informal casual: 꺼졌어
  • More formal polite: 꺼졌습니다
    The sentence as written is polite and very common in everyday conversation.