Learn Hardcore Esperanto: Mi havas pezan libron.

In Esperanto, all direct objects must take the accusative ending -n. Since libro is the direct object of the verb havas (“have”), it becomes libron. This marking clearly distinguishes the object from the subject.

Why does the adjective peza change to pezan when describing libro?

When an adjective directly modifies a noun that is in the accusative case, it must also take the accusative ending -n to agree with that noun. Since libro appears as libron, its modifier peza becomes pezan to show that they are linked in case.

Is the word order in Esperanto flexible because of these case endings?

Yes, the use of case endings like -n makes Esperanto’s word order quite flexible. Although the standard is subject–verb–object (SVO), the clear marking of grammatical roles allows for rearrangements without causing confusion. Nevertheless, sticking to the typical order often makes sentences clearer and more natural.

Could I say "Mi havas libron peza" instead of "Mi havas pezan libron"?

While the case endings do make it clear which words are connected, the conventional practice in Esperanto is to place adjectives before the noun they modify. Although "Mi havas libron peza" might be understood because of the accusative markers, the standard form "Mi havas pezan libron" is preferred by speakers to maintain clarity and natural flow.

How would this sentence change if I were talking about multiple books?

For plural objects, both the noun and its adjective take plural endings. The singular libro becomes plural libroj, and when it is the direct object, it takes the accusative ending to become librojn. Similarly, peza becomes pezaj in plural, and with the accusative it turns into pezajn. So the sentence for multiple books would be: "Mi havas pezajn librojn", meaning “I have heavy books.”