siktagi joyonghan geol bonikka modu baebureunga bwa.

Yes, literally it does, but it is a very natural kind of shorthand.

Here 식탁 does not just refer to the physical table. It can stand for the dining table situation or the people at the table. English does something similar:

• The room went quiet.
• The table is still eating dessert.

So 식탁이 조용한 걸 보니까 is basically saying seeing that the table is quiet = seeing that things at the table are quiet / no one is really talking or eating much.

The particle 이/가 marks 식탁 as the subject of the observed fact: the table is quiet.

Using 이 here feels natural because the speaker is noticing a specific piece of evidence and using it to make a guess.

• 식탁이 조용한 걸 보니까... = Seeing that the table is quiet...

If you used 식탁은, it would sound more like setting up a topic or contrast:

• 식탁은 조용한 걸 보니까... = As for the table, seeing that it’s quiet...

That is possible in some contexts, but 식탁이 is the more straightforward choice here.

조용한 걸 is a shortened colloquial form of 조용한 것을.

Breakdown:

• 조용하다 = to be quiet
• 조용한 = quiet / that is quiet
  
  • this is the adjective in noun-modifying form
• 것 = thing, fact
• 을 = object marker

So 조용한 것을 보니까 literally means seeing the fact that it is quiet.

In natural English, you would not translate it word-for-word. It just works like:

• seeing that it’s quiet
• judging from how quiet it is

Because Korean often contracts common spoken forms.

• 것을 → 걸
• 것이 → 게
• 것은 → 건

So:

• 조용한 것을 보니까 → 조용한 걸 보니까

This is very common in everyday speech and writing that sounds conversational. The meaning does not change.

Because it is modifying 것.

In Korean, descriptive verbs/adjectives change form before a noun:

• 조용하다 = to be quiet
• 조용한 = quiet, that is quiet

So:

• 조용한 것 = the quiet thing / the fact that it is quiet

This is the same pattern you see in:

• 예쁜 꽃 = pretty flower
• 바쁜 사람 = busy person
• 큰 집 = big house

Here the noun being modified is 것, which often means the fact that...

-걸 보니까 means something like:

• seeing that...
• judging from the fact that...
• from what I can see...

It shows that the speaker is making a conclusion based on what they observe.

So:

• 식탁이 조용한 걸 보니까
  = Seeing that the table is quiet
  = Judging from how quiet the table is

This pattern often introduces the evidence for the speaker’s guess.

Yes, you could, but the nuance is a little different.

• 조용해서 = because it is quiet
• 조용한 걸 보니까 = seeing that it is quiet / judging from the fact that it is quiet

So -걸 보니까 makes the speaker sound more like they are looking at evidence and inferring something.

Compare:

• 비가 와서 안 가. = I’m not going because it’s raining.
• 비가 오는 걸 보니까 우산이 필요하겠다. = Seeing that it’s raining, I guess I’ll need an umbrella.

In your sentence, 걸 보니까 is a good fit because the speaker is observing the quiet table and then guessing why.

Here it means everyone.

모두 can mean:

• all
• everyone
• everything

The exact meaning depends on context.

In 모두 배부른가 봐, the natural interpretation is:

• Everyone must be full
• Looks like everyone is full

You could also think of it as 모두가 with the subject marker omitted, which is very common in Korean conversation.

배부른가 봐 means I guess they’re full or it looks like they’re full.

Breakdown:

• 배부르다 = to be full
• 배부른가 = whether they are full / are they full?
• 보다 in this grammar pattern = to seem / to appear
• -ㄴ가 보다 → -ㄴ가 봐 in casual speech

So 배부른가 봐 is not a direct statement like they are full. It is an inference based on evidence.

Because 배부르다 is a descriptive verb/adjective, and the standard form with 보다 is:

• Adjectives: -ㄴ/은가 보다
• Action verbs: -나 보다
• Nouns + 이다: -인가 보다

So:

• 배부르다 → 배부른가 보다
• casual: 배부른가 봐

Examples:

• 바쁜가 봐. = I guess they’re busy.
• 먹나 봐. = I guess they’re eating.
• 학생인가 봐. = I guess they’re a student.

No, -가 봐 shows that the speaker is not stating a confirmed fact. They are making a guess from evidence.

It is like:

• I guess...
• it seems...
• looks like...
• must be... sometimes, depending on tone

So 모두 배부른가 봐 means the speaker is reasonably confident, but still inferring rather than directly knowing.

If they were completely direct, they might say:

• 모두 배부르다. = Everyone is full.

But 배부른가 봐 is softer and more observational.

Because it comes from 보다, which is still treated as a separate word in this grammar pattern.

So the standard spacing is:

• 배부른가 봐
• 학생인가 봐
• 오나 봐

Not:

• 배부른가봐
• 학생인가봐

In informal online writing, people sometimes ignore the space, but the standard written form keeps it separate.

It is casual/plain speech.

The ending -봐 here is informal, so this sentence would sound natural when speaking to:

• friends
• family
• someone younger
• yourself

A more polite version would be:

• 식탁이 조용한 걸 보니까 모두 배부른가 봐요.

If you want to be polite and also honor the people being talked about, you might say:

• 식탁이 조용한 걸 보니까 모두 배부르신가 봐요.

So the original sentence is natural, but definitely on the casual side.