oneureun hoeuiga issgeodeun, najunge jeonhwahalge.

-은 is the topic marker. Here it sets the frame as as for today or today. It can also carry a slight contrastive feeling, like today, at least, I have a meeting.

Korean time words are often used with no particle at all, so 오늘 회의가 있거든 is also possible. But 오늘은 sounds a bit more like the speaker is setting up the situation first.

With 있다, Korean usually uses an existence pattern:

• 회의가 있다 = there is a meeting

In context, that naturally means I have a meeting.

So Korean does not use the same structure as English have here.
회의를 있다 would be wrong. The particle 가 marks 회의 as the thing that exists.

Because 있다 normally takes the thing that exists as its subject.

So:

• 회의가 있다 = a meeting exists / there is a meeting

That is the standard pattern. If you changed it to 회의는, it would sound more contrastive, like as for the meeting... or the meeting, though..., which is a different nuance.

Here -거든 gives background or explanation before the main point. It is similar to:

• you see
• the thing is
• because

So the feeling is something like:

I have a meeting today, so I’ll call later.

It sounds conversational and explanatory, not just like a flat statement of cause.

Not exactly. All of them can relate to a reason, but the nuance is different.

• -아서/어서: neutral, straightforward reason
• -니까: more direct or explicit reason
• -거든: explanatory, conversational, often giving background the listener should know

So 오늘은 회의가 있어서 나중에 전화할게 is also very natural.
But 있거든 sounds a bit more like you know, I have a meeting today, so...

Yes. In this sentence, -거든 is connecting the first clause to the second one.

• 오늘은 회의가 있거든,
• 나중에 전화할게.

The first part gives the background reason, and the second part gives the speaker’s response or plan. So 있거든 is not the final ending here; it leads into the next clause.

It is:

• 전화하다 = to call / to telephone
• -ㄹ게 = a future-looking ending showing the speaker’s intention or promise

So 전화할게 means more than just a neutral future. It often sounds like:

• I’ll call
• I’ll call you later, okay
• I’ll make sure to call later

It is very natural when the speaker is reassuring the listener.

전화할게 is more directed toward the listener. It often sounds like a promise, offer, or decision made with the other person in mind.

전화할 거야 is a more neutral statement of future plan or intention.

So:

• 전화할게 = I’ll call you later
• 전화할 거야 = I’m going to call later

In this sentence, 전화할게 fits well because the speaker is responding to the situation and reassuring the listener.

Yes, but that is completely normal in Korean.

In English, you would usually say:

• I have a meeting today
• I’ll call later

In Korean, the subject is often omitted when it is obvious from context. Here, the speaker is clearly the one who has the meeting and the one who will call, so 나는 does not need to be said.

The listener is understood to be the person being called, so Korean leaves it out.

If you wanted to make it explicit, you could say:

• 나중에 너한테 전화할게 = I’ll call you later

But in normal conversation, leaving it out sounds more natural when the context is clear.

It is casual speech.

• 있거든
• 전화할게

There is no 요, so this would normally be used with a friend, someone younger, or someone you speak casually with.

A polite version could be:

오늘은 회의가 있거든요, 나중에 전화할게요.

A slightly more neutral polite version would be:

오늘은 회의가 있어서 나중에 전화할게요.

나중에 just means later. The exact time depends on context.

In this sentence, it most naturally means after the meeting or when I’m free later. By itself, it does not give a precise time.