Questions & Answers about Kötturinn skríður undir rörið.
Why does undir take the accusative case in this sentence?
Because the cat is actively moving to a destination. The preposition undir takes the dative case when describing a fixed location (where something already is), but it takes the accusative case when describing movement towards a new place.
How can I tell rörið is in the accusative case? It looks exactly like the dictionary form.
Rör (pipe) is a neuter noun. A very helpful rule in Icelandic is that all neuter nouns look exactly the same in both the nominative (subject) and accusative (destination) cases. It is accusative here because of the motion, even though the spelling didn't change.
The verb is skríður. What is its infinitive form?
The infinitive is skríða (to crawl or creep). Because the subject is a third-person singular noun (the cat), we drop the -a and add the present tense ending -ur.
As a quick refresher, how does köttur become "the cat"?
In Icelandic, "the" is attached to the end of the noun rather than being a separate word. Köttur is a masculine noun in the nominative case, so we add the masculine definite suffix -inn to get kötturinn.