hyudaeponeul kyeon chaero jamdeureosseoyo.

켠 comes from the verb 켜다, which means to turn on.

In this sentence, 켠 is the adnominal (modifier) form of 켜다, used before 채로. So:

• 켜다 → to turn on
• 켠 → turned on / in the state of having turned on

This form is a little irregular-looking because 켜다 does not become 켜은. It contracts to 켠.

So 휴대폰을 켠 채로 means something like:

• with the phone left on
• while the phone was still on

채로 means in the state of, while remaining, or without changing that state.

It is used when something stays as it is, and another action happens during that unchanged state.

So in 휴대폰을 켠 채로 잠들었어요, the idea is:

• the phone was turned on
• that state continued
• then the person fell asleep

A useful way to think about -ㄴ/은 채로 is:

• leave something as it is and do something else

More examples:

• 문을 연 채로 나갔어요.
  He left with the door open.

• 불을 켠 채로 잤어요.
  I slept with the light on.

This is a very common question.

Compare:

• 휴대폰을 켜고 잠들었어요.
• 휴대폰을 켠 채로 잠들었어요.

Both can be translated similarly, but the nuance is different.

-고 simply connects actions:

• turned on the phone and fell asleep

It can sound like a sequence of events.

-ㄴ/은 채로 emphasizes the continuing state:

• fell asleep with the phone left on
• fell asleep while it was still on

So 채로 is better when the important point is not just that the phone was turned on first, but that it remained on when the person fell asleep.

It is 휴대폰을 because 휴대폰 is the object of 켜다 (to turn on).

• 휴대폰을 켜다 = to turn on the phone

Even though the whole phrase is 휴대폰을 켠 채로, the object marker 을 stays because 휴대폰 still belongs to the verb 켜다 inside that modifier phrase.

So the structure is basically:

• [휴대폰을 켠] 채로
  in the state of having turned on the phone

If you used 휴대폰이, it would suggest a different structure and would not fit naturally with 켜다 in this sentence.

잠들다 means to fall asleep, while 자다 means to sleep.

So:

• 잠들었어요 = fell asleep
• 잤어요 = slept

In this sentence, 잠들었어요 focuses on the moment or event of drifting off to sleep while the phone was still on.

If you said 휴대폰을 켠 채로 잤어요, that would mean:

• I slept with my phone on

That is also possible, but it focuses more on the sleeping state in general.

So the original sentence is specifically about falling asleep in that condition.

Yes. In many cases, 채 and 채로 are both used, and the meaning is very similar.

For example:

• 문을 연 채 있었어요.
• 문을 연 채로 있었어요.

Both are natural.

In everyday speech, 채로 is very common and often feels a bit clearer or more complete. For learners, it is usually safest to learn and use -ㄴ/은 채로 first.

So:

• 휴대폰을 켠 채 잠들었어요
• 휴대폰을 켠 채로 잠들었어요

Both are possible, but the second is especially common and easy to recognize.

Both can be possible in Korean, but they are slightly different in nuance.

• 켠 채로 focuses on the fact that someone turned it on and left it that way.
• 켜진 채로 focuses on the resulting state: being on

So:

• 휴대폰을 켠 채로 잠들었어요
  I fell asleep after turning my phone on and leaving it on.

• 휴대폰이 켜진 채로 있었어요
  The phone was in the state of being on.

In your sentence, 켠 채로 sounds very natural because the speaker is talking about their own action and its continuing result.

The main verb 잠들었어요 is in the past tense polite form.

Breakdown:

• 잠들다 = to fall asleep
• 잠들었어요 = fell asleep

The earlier part, 켠 채로, is not a separate past-tense sentence. It is a modifier showing the state that existed when the main action happened.

So the overall sentence is past tense because the main event, falling asleep, happened in the past.

It is in the polite everyday style because it ends in -어요:

• 잠들었어요

This is natural for normal conversation, writing to someone politely, or speaking to someone you are not extremely close to.

Compare:

• 잠들었어요 = polite
• 잠들었어 = casual
• 잠들었습니다 = formal

So the sentence is polite, but not highly formal.

휴대폰 literally means a portable phone, and in modern Korean it normally means a cell phone / mobile phone.

It is one of the most common standard words for cell phone.

You may also hear:

• 핸드폰 — very common in speech, borrowed from English-style Korean
• 스마트폰 — smartphone

So in this sentence, 휴대폰 simply means cell phone or mobile phone.

Yes, very often. -ㄴ/은 채로 is a very useful pattern.

You can use it with many verbs to mean while remaining in that state or without changing it.

Examples:

• 신발을 신은 채로 들어왔어요.
  He came in with his shoes on.

• 창문을 닫은 채로 잤어요.
  I slept with the window closed.

• 가방을 멘 채로 앉아 있었어요.
  She was sitting with her bag still on.

So this sentence is a great example of a broader pattern you can reuse a lot.