Breakdown of eeokeoneul kyeossneunde gapjagi kkeojyeosseoyo.
Questions & Answers about eeokeoneul kyeossneunde gapjagi kkeojyeosseoyo.
-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.
-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.
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.