gyotongsagoga naseo gugeupchaga baro wasseoyo.

In Korean, 사고가 나다 is the natural expression for an accident happens / an accident occurs.
So 교통사고가 나다 is a set phrase meaning a traffic accident occurs.

This is different from English, where we usually use happen. In Korean, 나다 is often used with things like:

• 사고가 나다 — an accident happens
• 불이 나다 — a fire breaks out
• 문제가 나다 — a problem occurs

So even though 나다 often means to come out or to appear in other contexts, here it means to occur.

나서 comes from 나다 + -아서/어서.

So the structure is:

• 나다 — to happen, to occur
• 나서 — because it happened / and then it happened

In this sentence, 교통사고가 나서 connects the first event to the second one. It shows that the traffic accident happened, and as a result, the ambulance came right away.

Also, this is a normal contraction:

• 가다 → 가서
• 오다 → 와서
• 나다 → 나서

It can do both, and here both ideas fit.

교통사고가 나서 구급차가 바로 왔어요 can feel like:

• A traffic accident happened, so the ambulance came right away
• A traffic accident happened, and then the ambulance came right away

In this sentence, the causal meaning is especially strong, because the ambulance came because of the accident.

So -아서/어서 often links events in a natural sequence, and very often that sequence also implies cause.

Because there are two separate clauses, and each clause has its own subject.

• 교통사고가 나서 — the traffic accident is the subject of 나다
• 구급차가 바로 왔어요 — the ambulance is the subject of 오다

So the sentence is really made of two parts:

1. 교통사고가 났다
2. 구급차가 바로 왔다

Then they are connected with -서.

This is very normal in Korean.

Because 나다 takes the thing that occurs as its subject, not as a direct object.

So Korean says:

• 사고가 나다
• not 사고를 나다

That is just how this verb works. With 나다, the event or thing that happens is marked with 이/가.

바로 means right away, immediately, or directly.

Here it tells you that the ambulance came very quickly after the accident. It adds a sense of immediacy.

Compare:

• 구급차가 왔어요 — the ambulance came
• 구급차가 바로 왔어요 — the ambulance came right away

So 바로 emphasizes speed.

Because the sentence is describing something that already happened.

• 오다 — to come
• 왔어요 — came

The whole sentence is talking about a completed situation: the accident happened, and the ambulance came.

The ending -어요 also makes it polite, so this is a standard polite conversational style.

Korean does not have articles like English a, an, and the.

So 구급차가 can mean:

• an ambulance
• the ambulance

Which one sounds best depends on context and translation.

Korean usually leaves that kind of information unstated unless it really matters.

No. It only says that the ambulance came.

Korean often leaves out information that is not necessary or is obvious from context. So this sentence does not tell you:

• who called
• who was injured
• who was involved in the accident

It just states the chain of events.

Yes.

Same sentence in different speech levels:

• 교통사고가 나서 구급차가 바로 왔어. — casual
• 교통사고가 나서 구급차가 바로 왔어요. — polite
• 교통사고가 나서 구급차가 바로 왔습니다. — formal

The grammar is the same; only the ending changes depending on the situation and level of politeness.

Yes, the order is natural and important for the flow.

Korean usually puts the reason or background first, then the main result:

• 교통사고가 나서 — because a traffic accident happened
• 구급차가 바로 왔어요 — the ambulance came right away

So this order matches the usual Korean pattern of cause/background first, main event second. Reversing it would sound much less natural in ordinary speech.