i gaein samusireun bangeumi jaldwaeyo.

A very literal breakdown is:

• 이 = this
• 개인 사무실은 = as for this private office
• 방음이 = the soundproofing
• 잘돼요 = is done well / works well

So the sentence is structured more like:

As for this private office, the soundproofing is good / works well.

That is a very natural Korean way to express the idea.

Not exactly. It is better to think of this as topic + subject.

• 이 개인 사무실은: the topic of the sentence
• 방음이: the subject of the predicate 잘돼요

So Korean is saying:

As for this private office, the soundproofing is good.

English usually packs this into one subject, but Korean often separates the bigger topic from the thing being described.

개인 사무실 means private office or individual office.

Here, 개인 is originally a noun meaning individual or private person, but Korean often uses nouns directly in front of other nouns to modify them. So:

• 개인
  • 사무실
• literally something like individual/private office

This is very common in Korean. It is not an adjective in the same way English adjectives work, but it functions like a modifier here.

방음 means soundproofing or sound insulation.

It refers to blocking or reducing sound, especially between rooms or from outside noise. In this sentence, it is talking about how well the office keeps sound from passing through.

잘 is an adverb meaning well.

So:

• 되다 = to become, to be done, to work out
• 잘되다 = to go well, to be done well, to work well

In this sentence, 잘 adds the idea that the soundproofing is effective or good.

Without 잘, the sentence would not sound as natural for this meaning.

Here, 잘돼요 means something like:

• is done well
• works well
• is effective

So 방음이 잘돼요 does not mean that soundproofing is literally becoming something in the moment. It means the office is well soundproofed or the soundproofing works well.

Because 돼요 comes from 되어요, and that contracts to 돼요.

So the pattern is:

• 되다
• 되어요
• 돼요

되요 is a very common mistake, but standard Korean is 돼요.

This is a very common Korean pattern.

• 방음하다 = to soundproof something, to do the soundproofing
• 방음이 잘되다 = to be well soundproofed, for the soundproofing to work well

So 하다 would focus more on the action of doing the soundproofing, while 되다 here describes the result or state.

That is why 방음이 잘돼요 sounds natural for It’s well soundproofed.

잘돼요 is in the polite casual style, often called 해요체. It is very common in everyday conversation.

Other versions would be:

• 방음이 잘됩니다 = more formal
• 방음이 잘돼 = casual, non-polite
• 방음이 잘된다 = plain dictionary-style / written style

So the original sentence is polite and natural for normal conversation.

Yes, both can be used, but they feel slightly different.

• 방음이 좋아요 = the soundproofing is good
  This is a simple quality judgment.

• 방음이 잘돼요 = the soundproofing works well / has been done well
  This emphasizes effectiveness or the result.

• 방음이 잘되어 있어요 = it is in a well-soundproofed state
  This sounds a little more descriptive and state-focused.

In everyday speech, 방음이 잘돼요 is very natural and common. It gives the sense that the office is effectively soundproofed.