Learn Icelandic: Steinarnir eru mjög gamlir.

It starts with the masculine noun steinn (stone). First, we make it plural by adding -ar to get steinar (stones). Then, to say "the stones," we add the masculine plural definite ending -nir, which gives us steinarnir.

Why is the word for "old" spelled gamlir here instead of its dictionary form gamall?

In Icelandic, adjectives change their endings to match the noun they describe. Because steinarnir is a masculine plural noun, the adjective takes the masculine plural ending -ir.

Wait, shouldn't gamall plus the -ir ending be "gamallir"?

When an ending that starts with a vowel (like -ir) is added to certain adjectives like gamall, the second a drops out to make it easier to say. The double ll also simplifies to a single l. This pattern turns gamall into gamlir.

Does the word mjög (very) change to match the plural noun too?

No, it doesn't. Mjög is an adverb, and unlike adjectives, adverbs in Icelandic never change their form. It stays exactly the same no matter what gender or number it is paired with.