Learn Hardcore Esperanto: La libro estas granda.

Esperanto only has one article: la (the definite article). There is no separate indefinite article (like "a" or "an" in English). So, if you want to say "the book," you would say la libro, and if you mean "a book," you would simply say libro.

Why is the word order "La libro estas granda" instead of "Granda estas la libro"?

Esperanto allows relatively flexible word order. The most common pattern is Subject – Verb – Complement (la libro – estas – granda). You could say Granda estas la libro, but the usual, straightforward presentation is La libro estas granda.

What is the underlying verb form for "estas," and why is it used here?

The verb esti (to be) is conjugated in the present tense as estas for all persons (eg. mi estas, vi estas, li/ŝi/ĝi estas, etc.). In this sentence, estas is simply stating that the book exists in a state of being big, so La libro estas granda means "The book is big."

Why does "granda" end with -a?

In Esperanto, adjectives (words describing nouns) typically end in -a. Thus, granda is the adjective form meaning big. Esperanto uses consistent endings for parts of speech: -o for nouns, -a for adjectives, and -e for adverbs.

Do I need to change the ending of "granda" if the book is plural?

Yes. In Esperanto, if you want to talk about multiple books, you add -j to both the noun and the adjective for agreement. So, if you have multiple books, they are la libroj, and if they are big, you would say la libroj estas grandaj.