Learn Hardcore Spanish (Latin America): El libro está cubierto de polvo.

Because está comes from the verb estar, which we use to describe a temporary condition or state. Here, the book isn’t inherently dusty; it currently has a layer of dust. Using ser (so es) would imply a permanent or defining quality, which doesn’t fit for “covered in dust.”

What role does cubierto play in this sentence?

Cubierto is the past participle of cubrir acting like an adjective. In the construction estar + past participle, it describes the resulting state (the book is in a covered state). It’s not an action in progress but the condition of being covered.

Why do we say cubierto de polvo and not cubierto por polvo or cubierto con polvo?

Spanish often uses de after participles of coverage or filling to express “covered with” as a general material or substance. Con also works in many contexts (e.g. cubierto con pintura), but with dust or fine particles, de is more idiomatic. Por would suggest agency (“covered by someone or something doing the action”), which isn’t the case here.

Can I say El libro está polvoriento instead?

Yes, polvoriento is an adjective meaning “dusty.” It’s perfectly correct and more concise. The nuance is minimal:

El libro está cubierto de polvo emphasizes the layer of dust as a covering.
El libro está polvoriento focuses on the general dusty condition.

Why is polvo singular here?

Polvo is an uncountable noun in Spanish (like “dust” in English), so we treat it as a mass or “substance” rather than individual grains. Mass nouns remain singular.

Could I express the same idea with a passive voice like El libro fue cubierto de polvo?

Grammatically yes, but it changes the meaning.

El libro fue cubierto de polvo sounds like someone or something actively covered the book with dust at a specific moment (passive of action).
El libro está cubierto de polvo describes its current state. The first focuses on the action’s occurrence; the second on the resulting condition.