Learn Hardcore Icelandic: Hún keypti nýjar eldspýtur í búðinni, því gömlu eldspýturnar voru blautar. - She bought new matches at the store because the old matches were wet.

Hún keypti nýjar eldspýtur í búðinni, því gömlu eldspýturnar voru blautar.

She bought new matches at the store because the old matches were wet.

Breakdown of Hún keypti nýjar eldspýtur í búðinni, því gömlu eldspýturnar voru blautar.

Questions & Answers about Hún keypti nýjar eldspýtur í búðinni, því gömlu eldspýturnar voru blautar.

Why does nýjar end in -ar here?

Because eldspýtur is feminine plural, and in this sentence it’s the direct object of keypti (bought), so it’s in the accusative plural.
Adjectives agree with the noun in gender + number + case, so:

nýr (m.sg) / ný (f.sg) / nýtt (n.sg)
nýjar = feminine plural accusative (also the same form is used for feminine plural nominative in many adjective paradigms, but here the role is accusative because it’s the object)

So nýjar eldspýtur = new matches (as the thing she bought).

Why is it eldspýtur and not eldspýturnar in the first part?

eldspýtur is indefinite (matches, not the matches). In the first clause, she bought some new matches (not necessarily previously known/identified ones).
In the second clause we get eldspýturnar (the matches) because it refers to a specific set: the old matches she already had.

So the sentence contrasts:

nýjar eldspýtur = new matches (indefinite/new to the story)
gömlu eldspýturnar = the old matches (definite/known)

What’s going on with gömlu eldspýturnar—why is the adjective gömlu and not something like gömlur?