Learn Spanish (Latin America): Los libros viejos son baratos.

In Spanish, descriptive adjectives like viejos (old) almost always go after the noun they describe. So, instead of saying "old books" like we do in English, you say "books old" (libros viejos).

Why do both viejos and baratos end in -os?

Adjectives in Spanish have to match the gender (masculine or feminine) and number (singular or plural) of the noun they describe. Because libros is a masculine, plural noun, the adjectives describing it must also end in -os to match.

Does baratos have to match libros even though it comes after the verb son?

Yes! Even when an adjective comes after a verb like son (are), it still describes the main noun in the sentence. So baratos (cheap) must still be masculine and plural to match libros (books).

Why use son instead of están for "are"?

We use son (from the verb ser) because being cheap or old is treated as a basic characteristic or fact about these specific books. The verb estar is usually saved for temporary physical conditions, emotions, or locations.