Återbetalningen kommer nog idag.

Because återbetalningen is the definite singular form of the noun återbetalning.

• återbetalning = refund / repayment
• återbetalningen = the refund / the repayment

In this sentence, it refers to a specific refund that both speaker and listener already know about, so Swedish uses the definite form.

It is singular.

• en återbetalning = a refund
• återbetalningen = the refund
• återbetalningar = refunds
• återbetalningarna = the refunds

So the sentence is talking about one specific refund, not several.

Here kommer is the present tense of komma, which usually means to come.

In this sentence, it keeps that basic idea of coming / arriving. Swedish often uses the present tense for something in the near future, especially when there is a time word like idag.

So kommer idag is understood as something like is coming today or will arrive today.

Because komma here is being used as the main verb meaning come / arrive, not as a future marker.

Compare:

• Återbetalningen kommer idag. = The refund is coming/arriving today.
• Återbetalningen kommer att betalas ut idag. = The refund will be paid out today.

In your sentence, kommer has its normal lexical meaning, so att is not needed.

Nog usually means probably, I suppose, or I expect.

It adds uncertainty, but only a mild amount. The speaker sounds fairly confident, just not 100% sure.

So kommer nog idag suggests:

• will probably come today
• should come today, I think

It is a very common Swedish word in everyday speech.

This is normal Swedish word order in a main clause.

The pattern is:

• subject
  • finite verb
    • sentence adverb
      • other parts

So here:

• Återbetalningen = subject
• kommer = finite verb
• nog = sentence adverb
• idag = time expression

That gives:

Återbetalningen kommer nog idag.

This happens because Swedish is a V2 language, which means the finite verb normally comes in the second position in main clauses.

Not in normal standard Swedish main-clause word order.
Återbetalningen nog kommer idag sounds wrong.

The normal order is:

Återbetalningen kommer nog idag.

Since the finite verb usually has to be in second position, kommer must come before nog here.

Because time expressions often come later in the sentence, especially after a word like nog.

So this order sounds natural:

Återbetalningen kommer nog idag.

You can sometimes move idag for emphasis, for example:

Idag kommer återbetalningen nog.

But that sounds more marked or stylistically different. The version with idag at the end is the most neutral and natural here.

Yes. Idag and i dag are both accepted spellings.

• idag = more common in modern writing
• i dag = also correct

There is no difference in meaning here.

No. Idag is already an adverb meaning today, so no preposition is used.

Just say:

Återbetalningen kommer nog idag.

Not:

• på idag
• i idag

Swedish uses idag by itself.

A rough guide is:

OH-ter-beh-TAHL-ning-en

A few useful points:

• å sounds roughly like the o in British more or or
• stress falls on tal in betalning
• the g in -ning is not strongly pronounced as a full g sound

If you break it up, it is easier:

• åter
• betalning
• -en

So: åter-betalning-en

Yes, it is a normal and useful word. It literally means something like repayment / paying back.

Depending on context, Swedish can also use other words, but återbetalning is very common for money that is paid back, such as:

• a tax refund
• a reimbursement
• a returned payment

So in this sentence, it is a perfectly natural choice.