Learn Croatian: Nevjerojatno brzo je naučila hrvatski. - She learned Croatian incredibly fast.

They are functioning as adverbs here, modifying the verb naučila to describe how she learned. In Croatian, the adverb form of an adjective is almost always the same as its neuter singular form, which typically ends in -o (or sometimes -e).

Why is the clitic je placed after brzo? Doesn't it have to be the second word?

Clitics must go in the second position of a sentence, but that doesn't always mean the second literal word. Here, Nevjerojatno brzo acts as a single logical phrase occupying the first position. Therefore, the clitic je takes the second position right after that entire phrase.

Where is the word for 'she' in this sentence?

Croatian frequently drops subject pronouns like ona (she) because the verb usually gives you enough information. We know the subject is feminine because the past participle naučila ends in -a. If a male had learned it, the verb would be naučio.

Why does the sentence just say hrvatski instead of hrvatski jezik?

When talking about speaking, learning, or understanding a language, Croatians commonly drop the word jezik (language). The word hrvatski acts as a noun here. It is in the accusative case as the direct object of naučila, and because languages are inanimate, the accusative form looks exactly the same as the nominative.