Learn Hardcore German: Die Bibliothek ist vollständig leer.

Because Bibliothek is a feminine noun in German. In this sentence it’s the subject, so it takes the nominative case, feminine singular, which uses the article die.

Why are vollständig and leer left uninflected (no endings)?

They are used predicatively after the linking verb ist. Predicative adjectives and adverbs in German do not take any case endings—they remain in their base form when describing the subject via a verb like sein.

What part of speech is vollständig in this sentence?

Here vollständig functions as an adverb or intensifier modifying the adjective leer. It means “completely” or “totally,” so vollständig leer = “completely empty.”

Why is vollständig placed before leer?

Intensifiers (adverbs modifying adjectives) precede the adjective they modify. So you always say vollständig leer (intensifier + adjective), not leer vollständig.

Could you use another intensifier instead of vollständig?

Yes. Common synonyms are komplett, ganz, or total. For example:

Die Bibliothek ist komplett leer.
Die Bibliothek ist total leer.

Is there a nuance difference between vollständig leer and ganz leer?

Yes.

vollständig leer stresses absolute emptiness (“100% empty, no exceptions”).
ganz leer is a bit more colloquial, meaning “very empty” or “totally empty,” but it doesn’t carry quite the same sense of absolute completeness.

Why is ist used instead of hat?

In German you use sein (to be) to express a state or condition—here, emptiness. hat leer would be ungrammatical; you need ist leer to say “is empty.”

Could you say Die Bibliothek steht leer instead?

Yes, but it shifts the nuance. steht leer implies the building is unused or unoccupied (standing empty), whereas ist leer simply states its interior is empty.