Póstlúgan er föst aftur, svo ég hringi í húsvörðinn.

The basic noun is póstlúga. Icelandic usually puts the definite article on the end of the noun, so póstlúgan means the mail slot.

• póstlúga = a mail slot
• póstlúgan = the mail slot

Because the noun already ends in -a, adding the definite article gives the ending -an here.

Yes. It is made from:

• póst- = post, mail
• lúga = hatch, flap, small opening

So póstlúga is literally something like mail-flap or mail slot. Icelandic uses compound nouns very freely, and the last part usually determines the grammatical gender. Since lúga is feminine, póstlúga is feminine too.

Because Icelandic adjectives change form to agree with the noun in gender, number, and case.

The adjective is:

• fastur = masculine
• föst = feminine
• fast = neuter

Since póstlúgan is feminine singular nominative, the adjective must also be feminine singular nominative, so you get er föst.

Yes, historically they are related, but the meaning is not exactly the same in modern usage.

In Icelandic, fastur / föst / fast often means things like:

• fixed
• firm
• secure
• stuck

So in this sentence, föst means stuck or jammed, not quick.

Here aftur means again.

So the idea is that the mail slot is stuck again, not for the first time.

Be aware that aftur can also mean back in other contexts. Icelandic words like this often have a couple of related meanings, and the context tells you which one is meant.

Yes, adverbs like aftur can often move around a bit.

For example, both of these are possible:

• Póstlúgan er föst aftur
• Póstlúgan er aftur föst

They both mean roughly the same thing, but the emphasis and rhythm change slightly. In your sentence, er föst aftur sounds natural and idiomatic.

Because hringja is the infinitive, meaning to call / to ring, while hringi is the 1st person singular present tense.

• að hringja = to call
• ég hringi = I call / I am calling

So ég hringi is the correct form for I call or I’m calling.

When hringja means to call someone on the phone, Icelandic normally uses the pattern hringja í + accusative.

So:

• ég hringi í húsvörðinn = I call the caretaker

This is just the standard construction of the verb, and it is something learners have to memorize with hringja.

A useful contrast is:

• Síminn hringir = The phone is ringing
• Ég hringi í hana = I’m calling her

So hringja by itself can mean ring, but hringja í einhvern means call someone.

The basic noun is húsvörður, which means something like caretaker, building manager, superintendent, or sometimes janitor, depending on context.

In the phrase hringja í einhvern, the noun after í is in the accusative, so:

• húsvörður = nominative
• húsvörð = accusative

Then the definite article is added at the end:

• húsvörðinn = the caretaker

So the full form is there because the sentence means the caretaker, and because the verb pattern requires the accusative.

Here svo means so or and so, linking the first clause to the result in the second clause.

After svo in this use, normal clause order is fine:

• svo ég hringi í húsvörðinn

So the sentence structure is basically:

• The mail slot is stuck again, so I call / I’m calling the caretaker.

This is a natural way to connect two main ideas in Icelandic.

Icelandic often uses the present tense for something happening right now or for an immediate next action.

So ég hringi í húsvörðinn can mean:

• I’m calling the caretaker
• I’ll call the caretaker

depending on context.

If you want to make the future idea more explicit, Icelandic can also say:

• Ég ætla að hringja í húsvörðinn = I’m going to call the caretaker

But the simple present is very normal here.

Yes. In Icelandic, accents usually mark a different vowel sound, not just stress.

Roughly:

• ó in póst- sounds somewhat like the vowel in English go, but purer
• ú in hús- sounds like oo in food
• ö in vörð- is a separate vowel sound; English does not have a perfect match, but you can approximate it with a rounded version of the vowel in British bird

Also, ð in húsvörðinn is like the th in this.

Stress in Icelandic is usually on the first syllable, so roughly:

• PÓST-lú-gan
• HÚS-vör-ðinn