oneul jawonbongsajaga haengsajange iljjik wasseoyo.

-가 is the subject marker. It shows that the volunteer is the one who performed the action of coming.

So in this sentence:

• 자원봉사자가 = the volunteer (as the subject)
• 왔어요 = came

A learner often compares -가/-이 with -은/-는:

• 자원봉사자가 sounds like introducing or identifying who came
• 자원봉사자는 would make the volunteer the topic, often with a nuance like as for the volunteer...

So this sentence feels natural if you are simply stating what happened: The volunteer came early to the event venue today.

Because 에 marks a destination here.

• 행사장에 왔어요 = came to the event venue
• 행사장에서 would mean at the event venue and is used for where an action takes place

Compare:

• 행사장에 왔어요 = came to the venue
• 행사장에서 일했어요 = worked at the venue

With the verb 오다 (to come), if you are talking about the place someone comes to, 에 is the normal choice.

In Korean, time expressions often appear without a particle, especially in everyday speech.

So 오늘 by itself is very natural:

• 오늘 자원봉사자가... = Today, the volunteer...

You could also say 오늘은 if you want contrast or emphasis:

• 오늘은 자원봉사자가 행사장에 일찍 왔어요.
  • As for today / Today though, the volunteer came early to the venue.

But 오늘에 is generally not used in this kind of sentence.

왔어요 is the polite past tense of 오다 (to come).

Breakdown:

• dictionary form: 오다
• past stem: 왔-
• polite ending: -어요
• together: 왔어요 = came

This is a very common contraction:

• 오 + 았어요 → 왔어요

So 왔어요 means came in a polite, conversational style.

일찍 is an adverb, meaning early, so it usually goes before the verb it modifies.

Here it modifies 왔어요:

• 일찍 왔어요 = came early

This is the normal pattern in Korean:

• adverb + verb

Examples:

• 빨리 갔어요 = went quickly
• 조용히 말했어요 = spoke quietly
• 일찍 왔어요 = came early

• 행사 = event
• 행사장 = event venue / event hall / place where the event is held

The -장 part often refers to a place or site.

So:

• 행사에 왔어요 = came to the event
• 행사장에 왔어요 = came to the event venue

In your sentence, the focus is specifically on the place the volunteer came to.

Yes. Korean word order is fairly flexible, as long as the particles stay correct and the verb usually stays near the end.

Original:

• 오늘 자원봉사자가 행사장에 일찍 왔어요.

Possible variations:

• 자원봉사자가 오늘 행사장에 일찍 왔어요.
• 오늘 행사장에 자원봉사자가 일찍 왔어요.
• 자원봉사자가 행사장에 오늘 일찍 왔어요.

All of these can work, though some sound more natural depending on context.

The most important thing is:

• particles show each word’s role
• the verb usually comes at the end

Yes. Korean often omits the subject if it is clear from context.

So you could say:

• 오늘 행사장에 일찍 왔어요.

This could mean:

• (The volunteer) came early to the event venue today
• or someone else, depending on context

In your original sentence, 자원봉사자가 is included because the speaker wants to state clearly who came.

Both are polite, but they belong to different speech styles.

• 왔어요 = polite, everyday conversational style
• 왔습니다 = formal polite style

So:

• 오늘 자원봉사자가 행사장에 일찍 왔어요.
  • natural in normal conversation
• 오늘 자원봉사자가 행사장에 일찍 왔습니다.
  • more formal, like an announcement, report, or presentation

Neither is wrong. It depends on the situation.

Yes. 자원봉사자 is a standard and common word meaning volunteer.

Related words:

• 자원봉사 = volunteer work / volunteering
• 자원봉사자 = volunteer (person)

You may also hear 봉사자, but 자원봉사자 is clearer and more complete in many situations.

Examples:

• 자원봉사를 했어요. = I did volunteer work.
• 자원봉사자예요. = I’m a volunteer.

Yes, and it changes the nuance a little.

• 오늘 자원봉사자가 행사장에 일찍 왔어요.

  • a neutral statement: Today, the volunteer came early to the event venue.

• 오늘은 자원봉사자가 행사장에 일찍 왔어요.

  • adds topic/contrast: As for today, the volunteer came early to the event venue.

Using -은 on 오늘 can suggest a contrast like:

• maybe on other days they did not come early
• or today is being highlighted in comparison with another time

So both are grammatical, but the plain 오늘 version is more neutral.