Learn Icelandic: Kötturinn drepur fluguna.

The base word is the feminine noun fluga (a fly). Because it is the direct object being killed, it changes to the accusative case, becoming flugu. To say the fly, you add the feminine accusative definite article -na to the end, resulting in fluguna.

Why is the cat kötturinn instead of just köttur?

Icelandic attaches the word for the directly to the end of the noun. Köttur is a masculine noun acting as the subject of the sentence, so it gets the masculine nominative ending -inn, making it kötturinn.

Why is the verb spelled drepur instead of drepa?

Drepa is the dictionary or infinitive form meaning to kill. Because the subject doing the killing is kötturinn, which is third-person singular (equivalent to he, she, or it), the verb changes its ending to drepur.

Does the verb drepa always force the victim into the accusative case?

Yes, drepa is a verb that takes a direct object. The thing or person receiving the action directly must always be in the accusative case, which is why we see fluguna instead of the subject form.