Learn Icelandic: Jón pantaði pitsuna.

Pantaði comes from the verb panta (to order). For this very common group of verbs (Group 1 weak verbs), you drop the final -a and add -aði to make it past tense for singular subjects like I, he, she, or it. Since Jón is a he, we use pantaði.

Why is the word for pizza pitsuna here instead of the dictionary form pitsa?

Because the pizza is the direct object receiving the action of being ordered, it must be in the accusative case. The basic accusative form of the feminine noun pitsa is pitsu.

How does the "the" part of pitsuna work?

In Icelandic, the word for the is attached to the end of the noun. Since the noun is in the feminine accusative form (pitsu), we add the matching feminine accusative definite article, which is -na. Together, pitsu + -na makes pitsuna (the pizza).