Learn Icelandic: Skórnir eru gamlir en peysurnar eru nýjar. - The shoes are old but the sweaters are new.

The singular word for shoe is skór, and its plural is exactly the same: skór (shoes). Because it is a masculine noun, we add the masculine plural definite ending -nir to get skórnir (the shoes).

How does the definite ending work for peysurnar (the sweaters)?

The singular word is peysa (a sweater), which is a feminine noun. Its plural is peysur (sweaters). To say "the sweaters", we add the feminine plural definite ending -nar, making it peysurnar.

Why do the adjectives have different endings in this sentence (gamlir and nýjar)?

Adjectives in Icelandic change to match the gender and number of the noun they describe. Since skórnir (the shoes) is masculine plural, it takes the masculine plural adjective gamlir. Peysurnar (the sweaters) is feminine plural, so it takes the feminine plural adjective nýjar.

The dictionary word for old is gamall. Why does it look like gamlir here?

When we add the masculine plural ending -ir to gamall, the second 'a' and one of the 'l's drop out to make the word flow better when spoken. This kind of letter dropping is a very common pattern in Icelandic when adding endings to words.