Póstmaðurinn kemur snemma í dag og setur sendinguna við hurðina.

The ending -inn is the definite article attached to the noun, so póstmaðurinn means the postman.

In Icelandic, the is often not a separate word like in English. Instead, it is added to the end of the noun:

• póstmaður = a postman / postman
• póstmaðurinn = the postman

This is very common in Icelandic.

Kemur is the present tense, 3rd person singular form of the verb koma (to come).

So:

• ég kem = I come
• þú kemur = you come
• hann/hún/það kemur = he/she/it comes

Here, the subject is póstmaðurinn (the postman), which is singular, so kemur is the correct form.

Setja is the infinitive form, meaning to put / to place.
Setur is the present tense form used with he/she/it:

• að setja = to put
• hann setur = he puts

Since the subject is the postman, Icelandic uses the present-tense finite verb setur.

This works much like English:

• The postman comes early today and puts the package by the door.

In Icelandic, once the subject póstmaðurinn has been stated, it does not need to be repeated before the second verb if it is still the same subject.

So:

• Póstmaðurinn kemur ... og setur ...
• literally: The postman comes ... and puts ...

The word og simply means and.

Snemma is an adverb meaning early.

It describes when the postman comes:

• kemur snemma = comes early

It is not changing form here; it just functions as an adverb.

In Icelandic, í dag literally means in day, but as a set phrase it means today.

This is the normal expression:

• í dag = today
• á morgun = tomorrow
• í gær = yesterday

So even though English uses one word, Icelandic commonly uses this two-word phrase.

Sendinguna is the definite singular accusative form of sending.

Here is the basic idea:

• sending = a delivery / shipment / package
• sendingin = the delivery (subject form)
• sendinguna = the delivery (object form)

In this sentence, the package/delivery is the direct object of setur (puts), so Icelandic uses the accusative case.

That is why you see sendinguna, not sendingin.

Hurðina is the definite form of hurð (door), and it appears after the preposition við.

• hurð = door
• hurðin = the door
• hurðina = the door (accusative form)

The preposition við usually takes the accusative case, so Icelandic uses hurðina here.

So:

• við hurðina = by the door / at the door

In this sentence, við means something like by, next to, or at.

So:

• við hurðina = by the door

This is a common Icelandic preposition, but it can have several meanings depending on context. Here it is about location: the postman leaves the delivery at the door.

Yes, Icelandic word order is somewhat flexible, but this order is very natural.

• kemur snemma í dag = comes early today

The sentence first gives the verb, then the adverb snemma (early), then the time phrase í dag (today). That sounds smooth and ordinary.

You could also move things around for emphasis in other contexts, but this version is a very standard neutral order.

Because Icelandic usually attaches the definite article to the noun itself.

So instead of:

• the package
• the door

Icelandic often says:

• sendinguna
• hurðina

The the idea is built into the endings. This is one of the biggest differences from English.

Not exactly word-for-word, but it fits the context.

Sending can mean:

• shipment
• delivery
• consignment
• sometimes effectively package/parcels being delivered

In a sentence about a postman leaving something at the door, English might naturally say the package, even if Icelandic uses sendinguna.

So the most natural English translation may vary a little depending on context.

Yes. Icelandic uses the present tense here:

• kemur = comes
• setur = puts

Just like in English, the present tense can describe a current or scheduled event, or simply state what happens. The phrase í dag (today) gives the time reference, but the verbs themselves are still present tense.