Learn Icelandic: Skórnir eru mjög stórir.

The base word for shoe is the masculine noun skór. Interestingly, the plural for shoes is also just skór. To say the shoes, we add the masculine plural definite ending -nir to get skórnir.

Wait, so shoe and shoes are the exact same word? How do we know it is plural here?

Yes, skór can mean one shoe or multiple shoes. We know it is plural in this sentence because of three clues: the definite ending -nir, the plural verb eru (are), and the plural adjective stórir. If it were just one shoe, it would be skórinn er mjög stór.

Why does the adjective stórir end in -ir?

Adjectives in Icelandic change to match the noun they describe. Because skórnir is a masculine plural noun, the adjective stór (big) takes the masculine plural ending -ir.

Does the ending -nir mean the for all plural nouns?

No, -nir is specifically the ending for masculine plural nouns. If the noun were feminine, the plural definite ending would be -nar, and if it were neuter, it would be -in.