doseogwaneun joyonghaeseo jipjungi jal dwaeyo.

은/는 marks the topic of the sentence, so 도서관은 means something like as for the library or when it comes to the library.

In this sentence, the speaker is making a general comment about the library:

• 도서관은 조용해서... = The library is quiet, so...

If you used 도서관이, it would sound more like you are specifically identifying the library as the thing that is quiet, often in contrast to something else. Both can be grammatical in the right context, but 은/는 is very natural here for a general statement.

조용해서 comes from 조용하다 (to be quiet).

It is formed like this:

• 조용하-
  • -아서/어서 → 조용해서

Here, -아서/어서 means because or and so. So:

• 조용해서 = because it is quiet

This grammar connects the first part of the sentence to the result in the second part:

• 도서관은 조용해서 집중이 잘 돼요.
• Because the library is quiet, concentration goes well.

This happens because many Korean descriptive verbs ending in 하다 change in a regular way when followed by endings like -아서/어서.

For 하다 verbs/adjectives:

• 하다 → 해서

So:

• 조용하다 → 조용해서
• 공부하다 → 공부해서
• 편하다 → 편해서

So 조용해서 is just the normal connected form of 조용하다.

Literally, it means something like:

• concentration
  • subject marker
    • well
      • becomes / works out

So a very literal translation is:

• Concentration goes well
• It becomes easy to concentrate
• I can concentrate well

This sounds a little different from English, but it is very natural in Korean. The idea is not just I do concentration well, but rather the act of concentrating works smoothly / happens easily.

This is because the sentence uses 되다, not 하다.

Compare:

• 집중을 잘해요 = I concentrate well
• 집중이 잘 돼요 = Concentration goes well / It’s easy to concentrate

With 하다, 집중 is the object:

• 집중을 하다

With 되다, 집중 becomes the subject-like thing that works out / happens:

• 집중이 되다

So 집중이 잘 돼요 is a very common and natural Korean expression.

잘 usually means well, but in Korean it often has a broader sense than in English.

In 집중이 잘 돼요, 잘 means:

• well
• smoothly
• easily
• without difficulty

So the sentence does not only mean I concentrate skillfully. It more naturally means:

• I can concentrate well
• It’s easy to focus
• Focusing works well

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

집중을 잘해요

• More direct
• Focuses on the person’s ability
• I concentrate well

집중이 잘 돼요

• More natural in situations like this
• Focuses on the situation making concentration easy
• Concentration comes easily
• I can focus well

In your sentence, the library’s quietness creates good conditions for focusing, so 집중이 잘 돼요 fits very well.

되다 often means to become, but in many expressions it also means:

• to work
• to be possible
• to go well
• to happen successfully

So 집중이 되다 means:

• to be able to focus
• for concentration to happen
• for focusing to work

And:

• 집중이 잘 되다 = to be able to focus well / for concentration to go well

This usage is very common in Korean:

• 공부가 잘 돼요. = Studying goes well.
• 일이 잘 돼요. = Work is going well.
• 잠이 안 와요 / 잠이 잘 안 와요. is similar in feeling: something happens or does not happen easily.

돼요 is the polite, informal style, often called 해요체.

It is appropriate for:

• everyday conversation
• speaking politely to people you do not know well
• neutral, standard spoken Korean

The base form is:

• 되다 → 되어요 → 돼요

So 돼요 is just the contracted, natural spoken form of 되어요.

If you wanted a plain dictionary form, it would be:

• 도서관은 조용해서 집중이 잘 돼요. → 도서관은 조용해서 집중이 잘 된다.

Yes, you could.

Both -아서/어서 and -(으)니까 can express a reason, but they feel slightly different.

조용해서

• softer, more matter-of-fact
• simply links cause and result
• very natural here

조용하니까

• a bit more explicit: since/because
• can sound a little more explanatory or emphatic

So:

• 도서관은 조용해서 집중이 잘 돼요.
• 도서관은 조용하니까 집중이 잘 돼요.

Both are correct, but 조용해서 sounds especially smooth and natural in this kind of everyday statement.

It depends on context.

By itself, 도서관은 can mean either:

• the library in a specific situation
• libraries in general, as a type of place

Because 은/는 marks the topic, Korean often leaves this broad on purpose. So the sentence could mean:

• The library is quiet, so I can concentrate well or
• Libraries are quiet, so they’re good for concentrating

Context tells you which one the speaker means.