Greiðsludagurinn er á morgun.

It is a compound noun plus the definite article:

• greiðsla = payment
• greiðslu- = the form used in the compound
• dagur = day
• -inn = the

So greiðsludagurinn literally means the payment day, often understood as the payday or the payment date, depending on context.

In Icelandic, the definite article is usually attached to the end of the noun instead of appearing as a separate word.

So:

• dagur = day
• dagurinn = the day

That same pattern appears inside the compound:

• greiðsludagur = payment day
• greiðsludagurinn = the payment day

This is one of the biggest differences from English word structure.

Because the sentence is talking about a specific payment day, not just any payment day.

Compare:

• Greiðsludagur = a payment day / payment day
• Greiðsludagurinn = the payment day

Also, dagurinn is the nominative singular definite form of dagur, which fits here because it is the subject of the sentence.

Er is the present tense, 3rd person singular of vera, which means to be.

So:

• ég er = I am
• þú ert = you are
• hann / hún / það er = he / she / it is

In this sentence, Greiðsludagurinn er á morgun, er simply means is.

Á morgun is the normal Icelandic expression for tomorrow.

It is best learned as a fixed phrase:

• á morgun = tomorrow

Even though morgun is related to morning, the whole phrase á morgun functions like an adverb of time.

Because these two expressions mean different things:

• á morgun = tomorrow
• í morgun = this morning

This is a very common point of confusion for learners. The preposition changes the meaning of the whole expression, so it is best to memorize them as separate set phrases rather than trying to translate word by word.

Two useful things to notice:

• greiðsludagurinn is in the nominative, because it is the subject.
• morgun in á morgun is part of a fixed time expression, traditionally analyzed as accusative.

For a learner, the most practical approach is:

• learn greiðsludagurinn as the subject form
• learn á morgun as a whole expression meaning tomorrow

Yes. Icelandic word order is flexible, but the verb usually stays in second position in main clauses.

So you can say:

• Greiðsludagurinn er á morgun.
• Á morgun er greiðsludagurinn.

Both are natural, but the emphasis shifts a little:

• Greiðsludagurinn er á morgun starts with the subject
• Á morgun er greiðsludagurinn highlights the time first

A rough learner-friendly pronunciation would be:

GRAYTH-slu-da-gu-rinn er ow MOR-gun

A few important sound notes:

• ei in greiðslu- sounds roughly like ay
• ð sounds like the th in this
• á sounds like ow in cow
• Icelandic r is usually trilled or tapped

This approximation is only a guide, but it can help you get started.