Learn Hardcore Esperanto: Ili vojaĝas rapide kun amikoj.

In Esperanto, words ending in -e function as adverbs, describing how an action is performed. Because rapide describes how they travel, it is an adverb. If we wanted to describe the speed itself as a quality of something, we might use the adjective form rapida.

Why does kun amikoj not have the accusative ending -n?

The accusative -n typically marks the direct object of a verb. Here, kun amikoj is a prepositional phrase ("with friends"), so there is no need for the accusative. The noun amikoj indicates "friends," and because it's used with the preposition kun, it remains in its nominative form.

Why is it Ili and not some form like Ilia?

Ili is the personal pronoun meaning "they" in Esperanto. Ilia is the possessive pronoun meaning "their." Since the sentence is about "they travel," we need the subject pronoun, which is Ili.

Why is the verb vojaĝas rather than something like vojaĝo?

In Esperanto, -as is the present-tense verb ending, so vojaĝas means "they travel" (ongoing action). By contrast, vojaĝo is a noun meaning "a trip" or "a journey."

Can rapide come before vojaĝas, and would it change the meaning?

Esperanto has relatively flexible word order, so you could say Ili rapide vojaĝas kun amikoj. It wouldn’t change the core meaning of "they travel quickly with friends," although some subtle emphasis might shift depending on the word order.