Learn Icelandic: Kettlingurinn skríður undir sófann.

Undir takes the accusative case when it describes movement towards a new destination. Because the kitten is actively moving to get under the sofa, we use the accusative. If the kitten were already sleeping under the sofa (no change of location), undir would take the dative case instead.

Why does the word for the sofa (sófann) only have -nn at the end instead of -inn?

The base word is sófi, which is a weak masculine noun that ends in a vowel. In the accusative case, masculine nouns that end in a vowel just add -nn to mean "the". So sófi becomes sófann.

How does the word Kettlingurinn break down?

It comes from the masculine noun kettlingur (kitten). To say "the kitten", we add the masculine definite article -inn to the end, making it kettlingurinn. It is in the nominative case because the kitten is the subject doing the crawling.

If I wanted to say "I crawl", how does the verb skríður change?

The infinitive of the verb is skríða (to crawl). To say "I crawl", you drop the ending to make it ég skríð. The form skríður is used for he, she, or it (like the kitten here).