Ef póstlúgan er föst, kemur húsvörðurinn strax.

Ef means if.

It introduces a conditional subordinate clause:

• Ef póstlúgan er föst = If the mail slot is stuck

Then the main clause gives the result:

• kemur húsvörðurinn strax = the caretaker comes immediately / will come immediately

So the whole sentence follows the pattern If X, then Y.

They are both present-tense verb forms:

• er = is, from vera = to be
• kemur = comes, from koma = to come

Both are 3rd person singular, because each clause has a singular subject:

• póstlúgan is singular, so you get er
• húsvörðurinn is singular, so you get kemur

This is because Icelandic is a verb-second (V2) language in main clauses.

The first part of the sentence, Ef póstlúgan er föst, counts as the first element. After that, the finite verb in the main clause normally comes next:

• Ef póstlúgan er föst, kemur húsvörðurinn strax.

So the order is:

1. fronted if-clause
2. finite verb: kemur
3. subject: húsvörðurinn

If you put the main clause first, the order becomes more like what an English speaker expects:

• Húsvörðurinn kemur strax ef póstlúgan er föst.

Those endings show the definite article, which in Icelandic is usually attached to the end of the noun.

So:

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

and

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

So unlike English, Icelandic often says the-noun as one word rather than using a separate word like the.

Both are in the nominative singular.

Why?

• póstlúgan is the subject of er
• húsvörðurinn is the subject of kemur

In other words, each noun is the subject of its own clause, so nominative is exactly what you would expect.

The basic adjective is fastur, meaning stuck, jammed, or fast depending on context.

In Icelandic, adjectives change form to agree with the noun in gender, number, and case. Here, the noun póstlúgan is:

• feminine
• singular
• nominative

So the adjective becomes:

• föst = feminine singular nominative

That is why you get:

• póstlúgan er föst

A useful comparison:

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

Also, because this is a predicate adjective after vera, Icelandic still makes it agree with the subject.

Because Icelandic often uses the present tense where English might use will.

So this sentence can mean something like:

• If the mail slot is stuck, the caretaker comes immediately
• or more naturally in English, If the mail slot is stuck, the caretaker will come immediately

Icelandic does not have a future tense in the same way English does. Very often, the present tense covers both present and future meaning, depending on context.

Strax means immediately, right away, or at once.

It is an adverb, and here it modifies kemur:

• kemur húsvörðurinn strax = the caretaker comes immediately

Its position here is natural and neutral. Icelandic adverb placement can vary somewhat, but this sentence sounds normal and idiomatic.

You would usually say:

• Húsvörðurinn kemur strax ef póstlúgan er föst.

This version may be easier for English speakers to read at first, because the main clause keeps the familiar subject + verb order:

• Húsvörðurinn kemur

Comparing the two versions is helpful:

• Ef póstlúgan er föst, kemur húsvörðurinn strax.
• Húsvörðurinn kemur strax ef póstlúgan er föst.

They mean essentially the same thing. The main difference is word order.

Because the sentence begins with a subordinate clause:

• Ef póstlúgan er föst

and then moves into the main clause:

• kemur húsvörðurinn strax

In standard Icelandic writing, a comma is commonly used to separate a fronted subordinate clause from the main clause. So the comma here is normal punctuation.