Learn Icelandic: Hlauparinn hleypur mjög hratt.

It comes from the infinitive verb að hlaupa (to run). By removing the final -a and adding the suffix -ari, we create the masculine noun hlaupari meaning a runner. The -nn at the very end is the attached definite article, making it the runner.

Why does the verb become hleypur but the noun stays as hlaupari?

Strong verbs like að hlaupa experience a vowel shift in the present tense singular, changing the vowel au to ey. However, when we attach the -ari suffix to make an agent noun, we attach it directly to the base infinitive stem. That is why the noun keeps the original au vowel.

Are all nouns ending in -ari masculine, and how do they change in other cases?

Yes, agent nouns formed with the -ari suffix are always masculine. They follow a very regular weak declension pattern. In the singular, they end in -a for the accusative, dative, and genitive cases. If the runner were the object of a sentence, it would become hlauparann.

Why is the word for fast spelled as hratt here?