Learn Icelandic: Hundurinn hleypur hratt yfir brúna. - The dog runs fast over the bridge.

Because the dog is actively moving across the bridge. The preposition yfir takes the accusative case when it describes motion or a destination. If the dog were just standing over the bridge without moving across it, yfir would take the dative case instead.

What is the base word for brúna, and how is the definite article attached?

The base word is brú, which is a feminine noun meaning bridge. Because it comes after yfir in a sentence showing movement, it needs to be in the accusative case. The accusative definite ending for a feminine noun is -na, making it brúna (the bridge).

The infinitive form of the verb is hlaupa. Why is it spelled hleypur here?

This is because of a vowel shift. Hlaupa is a strong verb, and when you conjugate it for the third-person singular (he, she, or it), the vowel au shifts to ey. So, the dog runs becomes hundurinn hleypur.

Is hratt an adjective or an adverb here?

It acts as an adverb here because it describes how the dog runs. In Icelandic, you usually create an adverb by taking the neuter singular form of the adjective. The base adjective is hraður (fast), and its neuter form is hratt.