Learn Icelandic: Bóndinn á tuttugu kýr. - The farmer owns twenty cows.

The verb eiga (conjugated here as á) is used for literal ownership of physical things, property, and animals. The verb hafa is mostly used as a helping verb (like in "have done") or for abstract concepts (like having time), so it doesn't work for owning cows.

What is the infinitive of the verb á?

It comes from the verb eiga (to own). This is an irregular verb where the vowel changes in the singular present tense. For the third person (he/she/it), the form shrinks down to just the single letter á.

Does the number tuttugu need to match the accusative case of the cows?

No. In Icelandic, only the numbers one through four change their endings to match the gender and case of the noun. Since tuttugu (twenty) is higher than four, it never changes its form.

I thought kýr meant "a cow" in the singular. Why is it used for "cows" here?

The word kýr is a very irregular noun. While it is the singular nominative form for a cow, it is also the plural accusative form. Since the verb eiga requires its object to be in the accusative case, kýr is the correct plural form here.