changmuneul orae dada dumyeon bangi seuphaeseo gibuni dapdaphaeyo.

닫아 두다 is 닫다 + 아/어 두다.

This pattern often means:

• do something and leave it that way
• keep something in that state

So:

• 창문을 닫다 = to close the window
• 창문을 닫아 두다 = to leave the window closed / keep the window shut

Then -면 adds if/when, so 닫아 두면 means if/when you leave it closed.

If you said 닫으면, that focuses more on the act of closing it.
닫아 두면 focuses more on the continued state after closing.

Because 닫다 is a transitive verb: you close something.

So the thing being closed takes the object marker:

• 창문을 닫다 = close the window

If you used 창문이, you would usually need a different verb, such as:

• 창문이 닫히다 = the window closes / is closed

So in this sentence, 창문을 is correct because the window is the object of 닫아 두다.

Yes, here 오래 means for a long time.

So:

• 창문을 오래 닫아 두면 = if you leave the window closed for a long time

You could also say 오랫동안, and the meaning would be very similar:

• 창문을 오랫동안 닫아 두면

In everyday speech, 오래 is very common and natural.
It modifies the duration of the state/action: the window stays closed for a long time.

-면 can often be translated as if, when, or whenever, depending on context.

In this sentence, it expresses a general condition or repeated result:

• 창문을 오래 닫아 두면 ...
  = If/when you leave the window closed for a long time ...

It is not talking about just one specific future event only. It sounds more like a general truth or typical consequence:

• When you keep the window closed too long, this happens
• If you leave the window closed too long, this happens

So all three English ideas are possible, but if or when usually works best.

습해서 comes from 습하다 + -아서/어서, which links clauses and often means because or so.

So:

• 습하다 = to be humid
• 습해서 = because it is humid / being humid, ...

In this sentence:

• 방이 습해서 기분이 답답해요
• Because the room is humid, I feel stuffy/oppressed

A useful thing to notice: with 하다 words, -아/어서 often becomes 해서.

So:

• 습하다 → 습해서
• 조용하다 → 조용해서
• 편하다 → 편해서

Because the sentence has multiple clauses, and each clause can have its own subject.

Break it up like this:

1. 창문을 오래 닫아 두면
   = if you leave the window closed for a long time

2. 방이 습해서
   = because the room is humid

3. 기분이 답답해요
   = the feeling is stuffy/oppressed / I feel stuffy

So:

• 방이 is the subject of 습하다
• 기분이 is the subject of 답답하다

This is very normal in Korean. English often avoids repeating subjects in the same way, so it can feel unusual at first.

답답하다 is a very useful Korean word with a range of meanings. Depending on context, it can mean things like:

• stuffy
• oppressive
• stifling
• frustrating
• claustrophobic-feeling

In this sentence, because the room is humid and the window has been closed for a long time, 답답해요 is closer to:

• I feel stuffy
• I feel stifled
• It feels oppressive

So this is not just emotional frustration. It has a physical, bodily feeling too.

Yes, 답답하다 can be used without 기분이 if the meaning is clear from context.

For example:

• 답답해요. = I feel stuffy / frustrated / stifled.

Adding 기분이 makes it clearer that the sentence is describing your internal feeling or mood-like sensation.

Compare:

• 방이 답답해요 = The room feels stuffy
• 기분이 답답해요 = I feel stuffy / stifled
• 답답해요 = can mean either, depending on context

So 기분이 helps shift the focus from the room itself to the speaker’s felt experience.

Korean often leaves out subjects when they are obvious from context.

Here, the person feeling 답답해요 is understood from the situation, usually I or we depending on context.

So even though the sentence does not say 저는, it naturally implies something like:

• (저는) 창문을 오래 닫아 두면 ... 기분이 답답해요.

Adding 저는 is possible if you want emphasis or contrast, but it is not necessary.

This is very common in Korean and can feel strange to English speakers, since English usually wants an explicit subject.

Yes, there is a nuance difference.

• 창문을 오래 닫으면
  can sound like if you close the window for a long time, but it is a little less precise about the continuing state

• 창문을 오래 닫아 두면
  clearly means if you close it and leave it that way for a long time

Because this sentence is about the result of the window staying closed over time, 닫아 두면 is the more natural and clearer choice.

So if you want to express leave it closed, 아/어 두다 is very useful.

This is a common pronunciation question.

닫아 is written with ㄷ at the end of the stem, but when followed by a vowel, the sound links into the next syllable.

So 닫아 is pronounced roughly like 다다.

That means:

• 닫아 두면 sounds roughly like 다다 두면

This is normal Korean sound linking.
The spelling still reflects the base verb 닫다, but the pronunciation changes in connected speech.