oneureun hoeuiga issgeodeun, najunge jeonhwahalge.

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

Breakdown of oneureun hoeuiga issgeodeun, najunge jeonhwahalge.

• ~가: subject particle
• ~은: topic particle
• 오늘: today
• 있다: to exist

Questions & Answers about oneureun hoeuiga issgeodeun, najunge jeonhwahalge.

Why is 오늘은 marked with -은?

-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.

Why is it 회의가 있다 and not something like 나는 회의를 있다?

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.

Why is the particle 가 used after 회의?

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.

What exactly does 있거든 mean here?