seonkeurimeul an bareun chaero badasga-e orae isseossdeoni pari apasseo.

Because 채로 attaches to a form that describes a state.

Here, 바르다 becomes 바른 before 채로:

• 바르다 = to apply
• 바른 = having applied / applied
• 바른 채로 = in the state of having applied
• 안 바른 채로 = in the state of not having applied

So 선크림을 안 바른 채로 means without having put on sunscreen or while still not wearing sunscreen.

Using 바르 채로 would be ungrammatical.

채로 means something like:

• in the state of ...
• while remaining ...
• without changing that condition

So in this sentence:

• 선크림을 안 바른 채로 = remaining in the state of not having applied sunscreen

It emphasizes that the person stayed in that condition and then did something else.

Compare:

• 선크림을 안 바르고 바닷가에 있었어.
  = I didn’t put on sunscreen and was at the beach.
• 선크림을 안 바른 채로 바닷가에 있었어.
  = I was at the beach while still not wearing sunscreen.

The second one sounds a bit more like the unchanged state is important.

Because 바르다 takes an object: you apply something.

So:

• 선크림을 바르다 = to apply sunscreen
• 약을 바르다 = to apply medicine
• 크림을 바르다 = to apply cream

Even though the verb is inside a modifier phrase (안 바른 채로), 선크림 is still the object of 바르다, so it keeps 을.

They can be similar, but the nuance is different.

안 바르고

This is a simple connection:

• without applying / not applying and...

Example:

• 선크림을 안 바르고 나갔어.
  = I went out without applying sunscreen.

안 바른 채로

This emphasizes the continuing state:

• while remaining not applied
• in the condition of not having applied it

Example:

• 선크림을 안 바른 채로 바닷가에 오래 있었어.
  = I stayed at the beach a long time while still not wearing sunscreen.

So 채로 often feels a little more descriptive and highlights the resulting condition more strongly.

바닷가 means beach, seaside, or shore.

Here, 에 marks the place where someone exists/stays:

• 바닷가에 있었어 = was at the beach

This is because 있다 usually takes 에 for location:

• 집에 있어 = I’m at home
• 학교에 있었어 = I was at school
• 바닷가에 오래 있었어 = I stayed at the beach for a long time

So 에 is natural because the sentence is talking about being/staying somewhere.

Break it down like this:

• 오래 = for a long time
• 있었- = was / stayed
• -더니 = because / and then / as a result of something I experienced or observed

So 오래 있었더니 means roughly:

• because I stayed there for a long time
• after being there for a long time
• I stayed there a long time, and as a result...

In this sentence, it connects the situation to the result:

• 바닷가에 오래 있었더니 팔이 아팠어.
  = Because I stayed at the beach for a long time, my arms hurt.

-더니 often implies a result based on something the speaker personally experienced or noticed.

So:

• 바닷가에 오래 있었더니 팔이 아팠어.
  = I stayed at the beach a long time, and then my arms hurt / as a result my arms hurt.

It has a slightly more experiential, observational feel.

Compare with -아서/-어서

• 바닷가에 오래 있어서 팔이 아팠어.
  = My arms hurt because I was at the beach for a long time.

This is also possible, but -더니 feels a bit more like:

1. first event happened,
2. then the speaker noticed the result.

That makes it very natural in storytelling or personal experience.

Because with 아프다 (to hurt / to be painful), the body part is usually marked with 이/가, not 을/를.

So:

• 팔이 아프다 = my arm hurts / the arm is painful
• 머리가 아프다 = my head hurts
• 배가 아프다 = my stomach hurts

In Korean, the body part is treated as the thing that is in the painful condition, so 이/가 is used.

Using 팔을 아팠어 is incorrect.

Because the speaker is talking about what happened after staying at the beach.

• 아프다 = to hurt
• 아팠어 = hurt / was painful

The whole sentence is describing a past experience:

• I didn’t put on sunscreen,
• I stayed at the beach a long time,
• my arms hurt.

So past tense is natural.

It can mean my arm hurt or my arms hurt, depending on context.

In Korean, body parts are often used in the singular even when English might prefer the plural.

So:

• 팔이 아팠어 can naturally refer to my arms hurt
• If someone really wants to emphasize both arms, they might say 양팔이 아팠어 = both arms hurt

In your sentence, English would often translate it as my arms hurt, since sun exposure usually affects both.

It is casual/informal speech because it ends in 아팠어.

Casual:

• 팔이 아팠어.

Polite:

• 팔이 아팠어요.

A polite version of the full sentence would be:

• 선크림을 안 바른 채로 바닷가에 오래 있었더니 팔이 아팠어요.

So the original sentence sounds like something you would say to a friend, family member, or someone you speak casually with.

Yes. Even though the sentence literally says my arms hurt, the context strongly suggests something like sunburn or pain from too much sun exposure.

Because of:

• 선크림을 안 바른 채로 = without sunscreen
• 바닷가에 오래 있었더니 = after staying at the beach for a long time

a listener would probably understand that the speaker’s arms hurt from the sun.

Korean often leaves that kind of detail unstated when the context makes it obvious.

Yes. 선크림 is very common and means sunscreen or sun cream.

You may also see:

• 자외선 차단제 = UV blocker / sunscreen
  This is more formal or product-like.
• 썬크림 is sometimes seen, but 선크림 is the standard spelling.

In everyday conversation, 선크림 is the most natural word for many situations.