Learn Icelandic: Kötturinn borðar alltaf úti.

Borða (to eat) is a Group 1 weak verb. In the present tense, these verbs end in -ar for all singular subjects (I, you, he/she/it). Since kötturinn (the cat) is a "he/she/it", we use borðar.

If there were multiple cats, how would the verb borðar change?

If the subject is plural (we, you all, they), Group 1 verbs drop the -ar and look exactly like their dictionary form. So if we said "The cats always eat outside", the verb would just be borða instead of borðar.

What does the -inn at the end of Kötturinn do?

In Icelandic, the word for "the" is attached to the end of the noun instead of coming before it. Köttur means "a cat", and adding the masculine ending -inn makes it Kötturinn (the cat).

Why does the sentence use úti for "outside" instead of út?

Icelandic has different words for locations depending on whether there is movement. You use úti when someone or something is already outside and staying there (stationary), like the cat eating here. You would use út to describe motion, like going or running outside.