Húsvörðurinn opnar póstlúguna þegar hún festist.

In Icelandic, the definite article is usually a suffix, not a separate word.

• húsvörður = caretaker / janitor

• húsvörðurinn = the caretaker

• póstlúga = mail slot / letter flap
• póstlúguna = the mail slot

The exact form of that suffix changes depending on gender, case, and number, which is why you see -inn in one word and -na in the other.

Because póstlúguna is the accusative singular definite form.

It is the direct object of opnar (opens), and the verb opna takes an accusative object.

So:

• póstlúgan = the mail slot as a subject (nominative)
• póstlúguna = the mail slot as an object (accusative)

That is a very common Icelandic pattern: the form of the noun changes depending on its grammatical role.

The main clue is case.

• húsvörðurinn is in the nominative, which is the normal case for the subject.
• póstlúguna is in the accusative, which is commonly used for the direct object.

So even if English relies heavily on word order, Icelandic often makes the relationship clearer through endings.

Here hún means it, not she in the natural English sense.

Icelandic nouns have grammatical gender, and póstlúga is feminine. Because of that, a pronoun referring back to it must also be feminine:

• masculine noun → hann
• feminine noun → hún
• neuter noun → það

So hún refers to póstlúgan/póstlúguna simply because that noun is grammatically feminine.

festist is the 3rd person singular present tense of festast.

In this sentence it means something like:

• gets stuck
• becomes stuck
• jams

So hún festist means it gets stuck.

This is a very useful verb to learn because it often describes something becoming caught, jammed, or stuck.

They are related, but not the same.

• festa = to fasten, attach, fix something
  
  • usually transitive: someone fastens something
• festast = to get stuck, become attached, get caught
  
  • usually intransitive: something gets stuck

The -st ending often creates a verb with a different voice or meaning. In this sentence, festist does not mean that the mail slot is actively doing something to something else; it means the mail slot gets stuck.

þegar here means when.

It introduces a subordinate clause:

• Húsvörðurinn opnar póstlúguna = the caretaker opens the mail slot
• þegar hún festist = when it gets stuck

Depending on context, þegar can sometimes feel like when or whenever in English. In this sentence, the sense is very naturally understood as a repeated situation: when/whenever it gets stuck.

Most naturally, it sounds like a habitual or general statement.

Because both verbs are in the present tense:

• opnar = opens
• festist = gets stuck

the sentence is easily understood as:

• The caretaker opens the mail slot when/whenever it gets stuck.

So it often has the same feel as an English present-simple habit.

Because after a subordinating conjunction like þegar, Icelandic normally uses subordinate clause word order, not main-clause inversion.

So:

• þegar hún festist = correct, normal subordinate order

Icelandic main clauses often follow verb-second patterns, but subordinate clauses are usually more straightforward in their order.

For a learner, it is helpful to notice this contrast:

• main clause: Húsvörðurinn opnar póstlúguna
• subordinate clause: þegar hún festist

Yes. You could say:

Húsvörðurinn opnar póstlúguna þegar póstlúgan festist.

That is grammatically fine. But using hún is often more natural because it avoids repetition.

So the original sentence sounds smoother and more idiomatic.

opnar is the present tense singular form of opna used with a 3rd person singular subject such as húsvörðurinn.

Relevant forms are:

• ég opna = I open
• þú opnar = you open
• hann/hún/það opnar = he/she/it opens

Since húsvörðurinn is one person, the sentence needs the singular form opnar.