Húsvörðurinn kemur og opnar póstlúguna þegar hún er föst.

The ending -inn is the suffixed definite article in Icelandic, equivalent to the in English.

So:

• húsvörður = a caretaker / janitor
• húsvörðurinn = the caretaker / the janitor

Icelandic usually adds the definite article to the end of the noun instead of putting a separate word in front, unlike English.

There are two things happening here:

1. póstlúga means mail slot / letter flap
2. -na shows that it is:
   • definite: the mail slot
   • accusative singular feminine

It appears in the accusative because it is the direct object of opnar (opens).

So:

• póstlúga = a mail slot
• póstlúgan = the mail slot (nominative)
• póstlúguna = the mail slot (accusative)

Since the caretaker is opening the mail slot, Icelandic uses the object form póstlúguna.

In this sentence, hún means it, not she in the natural English sense.

That is because póstlúga is a feminine noun in Icelandic. When you refer back to a feminine noun, you use the feminine pronoun hún.

So grammatically:

• póstlúga = feminine noun
• therefore hún refers to it

English usually uses it for objects, but Icelandic uses pronouns according to grammatical gender.

So þegar hún er föst means when it is stuck, where it = the mail slot.

The adjective has to agree with the noun or pronoun it describes.

Here, föst goes with hún, which refers to póstlúga, a feminine singular noun.

The adjective fastur changes form depending on gender:

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

So because the mail slot is feminine, Icelandic uses föst.

In other words:

• hann er fastur = he/it (masculine) is stuck
• hún er föst = she/it (feminine) is stuck
• það er fast = it (neuter) is stuck

Here föst means stuck, jammed, or not opening properly.

The basic adjective is related to the idea of being fastened, fixed, or unable to move freely. In context, for a mail slot, the natural meaning is:

• stuck
• jammed

So þegar hún er föst is best understood as when it gets stuck or when it is jammed.

Icelandic often uses the present tense for actions that are:

• habitual
• repeated
• generally true
• part of a routine description

So this sentence sounds like a general statement:

The caretaker comes and opens the mail slot when it gets stuck.

It does not have to mean it is happening right now. It can mean this is what usually happens.

The verb forms are:

• kemur = comes
• opnar = opens

Both are 3rd person singular present, matching húsvörðurinn.

Because the subject is the same for both verbs.

In English we can say:

• The caretaker comes and opens the mail slot

We do not need to repeat the caretaker before opens.

Icelandic works the same way here:

• Húsvörðurinn kemur og opnar...

The subject húsvörðurinn applies to both kemur and opnar.

Þegar means when.

In this sentence it introduces a time clause:

• þegar hún er föst = when it is stuck

So the structure is:

• main clause: Húsvörðurinn kemur og opnar póstlúguna
• subordinate clause: þegar hún er föst

It tells you under what condition / at what time the action happens.

Be careful: þegar can also sometimes mean already in other contexts, but here it clearly means when.

Yes. This is a very normal sentence.

The main clause follows ordinary Icelandic word order:

• Húsvörðurinn = subject
• kemur og opnar = verbs
• póstlúguna = object

Then comes the subordinate clause:

• þegar hún er föst

So the whole sentence is built very naturally:

Subject + verb(s) + object + when-clause

If the sentence started with þegar, the word order in the main clause would change because of Icelandic verb-second patterns, but in the sentence as given, the order is straightforward.

Grammatically, hún could mean she in another context, but in this sentence the most natural reference is póstlúgan.

Why?

• póstlúga is feminine
• hún matches that feminine noun
• the phrase er föst makes good sense for a mail slot: it is stuck

So learners should understand hún here as a pronoun referring back to póstlúguna.

Hún is in the nominative case because it is the subject of the clause:

• hún er föst = it is stuck

Even though it refers back to póstlúguna, which appears earlier in the accusative as the object of opnar, the pronoun has its own role in its own clause.

So:

• in the main clause, póstlúguna is object → accusative
• in the subordinate clause, hún is subject → nominative

This is very common in Icelandic.

A rough learner-friendly breakdown is:

HÚS-vur-thur-inn

A few notes:

• ú is a long vowel, roughly like oo in food, though not exactly the same
• ö sounds somewhat like the vowel in some pronunciations of bird or her, but rounded
• ð is like the th in this
• the stress in Icelandic is usually on the first syllable

So the main stress is on HÚS.

You do not need perfect pronunciation immediately, but it helps to know that Icelandic nearly always stresses the first syllable.

Not exactly.

Póstlúga usually means a mail slot or letter flap—the opening where mail is pushed through a door or wall.

A mailbox in the sense of a box for receiving mail would more naturally be something like póstkassi.

So in this sentence, póstlúgan is specifically the flap/slot that can get stuck and be opened.