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:
So the order is:
If you put the main clause first, the order becomes more like what an English speaker expects:
Those endings show the definite article, which in Icelandic is usually attached to the end of the noun.
So:
and
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?
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:
So the adjective becomes:
That is why you get:
A useful comparison:
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:
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:
Its position here is natural and neutral. Icelandic adverb placement can vary somewhat, but this sentence sounds normal and idiomatic.
You would usually say:
This version may be easier for English speakers to read at first, because the main clause keeps the familiar subject + verb order:
Comparing the two versions is helpful:
They mean essentially the same thing. The main difference is word order.
Because the sentence begins with a subordinate clause:
and then moves into the main clause:
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.