bakkeseo keun soriga deullyeoseo gongsareul hana bwa.

밖에서 uses -에서, which often marks the place where something happens or the source/place something comes from.

Here, the loud sound is being heard from outside, so 밖에서 is very natural.

• 밖에서 큰 소리가 들려서 = Because I hear a loud sound from outside
• You can also think of it as outside / from outside

By contrast:

• 밖에 often means outside in a more general location sense, or appears in the pattern 밖에 없다
• 밖은 would mark outside as the topic, which is not what this sentence is doing

So in this sentence, 밖에서 is the normal choice.

큰 소리 literally means a big sound, but in natural English that usually becomes a loud noise or a loud sound.

크다 is the dictionary form meaning to be big / loud.
When Korean adjectives directly modify a noun, they change form:

• 크다 → 큰
• 큰 소리 = a loud sound

So:

• 크다 = to be big/loud
• 큰 소리 = a loud sound

This is just the normal adjective-before-noun form.

Because 소리 is the thing that is being heard, and with 들리다 it usually takes the subject marker -이/가, not the object marker -을/를.

• 소리가 들리다 = a sound is heard / I can hear a sound

This is different from 듣다:

• 소리를 듣다 = to listen to / hear a sound
• 소리가 들리다 = a sound is heard / can be heard

So 소리가 is correct because the sound is treated as the subject of 들리다.

This is a very common learner question.

• 듣다 = to listen / to hear in a more active sense
• 들리다 = to be heard / to can be heard in a more passive, natural-perception sense

In this sentence, the speaker is not actively trying to listen. They just notice that a loud noise is coming in from outside. So 들리다 fits better.

Compare:

• 밖에서 큰 소리를 들어요 = I listen to/hear the loud noise from outside
  (more active, less natural here)
• 밖에서 큰 소리가 들려요 = I hear a loud noise from outside / A loud noise can be heard from outside
  (natural here)

So 들려서 is used because the sound is simply reaching the speaker’s ears.

Here -아서/어서 means so / because / and so.

• 밖에서 큰 소리가 들려서 = Because I hear a loud noise outside / I hear a loud noise outside, so...

It connects the first clause to the speaker’s conclusion in the second clause:

• loud noise is heard outside
• therefore, the speaker guesses it is construction

So the sentence structure is basically:

[reason/evidence] + -서 + [conclusion]

공사 means construction, construction work, or sometimes repairs/work on a building or road depending on context.

공사를 하다 literally means to do construction work.

So:

• 공사 = construction work
• 공사를 하다 = to be doing construction

In this sentence, it usually means something like:

• They must be doing construction
• Looks like there’s construction work going on

Because 공사 is the object of 하다.

Many Korean nouns combine with 하다 to make a verb-like expression:

• 공부를 하다 = to study
• 운동을 하다 = to exercise
• 공사를 하다 = to do construction work

So 공사를 is simply:

• 공사
  • -를 (object marker)

It marks construction work as the thing being done.

No, it is not a real question here.

-나 보다 is a grammar pattern meaning:

• it seems...
• I guess...
• it looks like...
• must be... (in a guess/inference sense)

So:

• 공사를 하나 봐 = I guess they’re doing construction
• Looks like they’re doing construction

The speaker is making a guess based on evidence: the loud sound outside.

Even though 하나 looks like a question form, in -나 보다 it is part of a fixed grammar pattern for inference.

Because this comes from the grammar -나 보다, which attaches to verbs to show inference.

For 하다, that becomes:

• 하다 → 하나 보다
• in casual speech: 하나 봐

So the structure is:

• 공사를 하나 보다 = It seems they’re doing construction
• 공사를 하나 봐 = same meaning, but more conversational

해 봐 is completely different. It usually means try doing it.

And 하는 봐 is not the correct form for this grammar.

Yes.

In everyday speech, 보다 is often reduced:

• 하나 보다 → 하나 봐
• 오나 보다 → 오나 봐
• 비가 오나 보다 → 비가 오나 봐

So 공사를 하나 봐 is just the casual spoken form of 공사를 하나 보다.

The subject is omitted because it is obvious or unimportant.

In English, you might say:

• I guess they’re doing construction
• Looks like there’s construction going on

Korean often leaves out subjects when the meaning is clear from context. Here, the speaker does not need to say exactly who is doing it. It is naturally understood to mean something like:

• the workers
• people outside
• whoever is responsible

So Korean does not need an explicit they here.

It is casual/informal.

The ending -봐 here is informal speech, often used with friends, family, or in self-talk.

More polite versions would be:

• 공사를 하나 봐요
• 공사를 하나 봅니다 or 공사를 하는가 봅니다 in more formal styles

So the given sentence sounds like normal casual spoken Korean.

Yes, absolutely.

That is a very natural English translation. The Korean sentence is based on an inference from evidence, so several English versions work:

• I hear a loud noise outside, so I guess they’re doing construction.
• There’s a loud noise outside, so there must be construction going on.
• Sounds like they’re doing construction outside.

All of these capture the same basic meaning and nuance.