Learn Italian: Questi castelli sono grandi e famosi. - These castles are big and famous.

They both end in -i to match the plural noun castelli, but they belong to different adjective groups! Famosi comes from famoso, which has four different endings based on gender and number (-o, -a, -i, -e). Grandi comes from grande, which belongs to a group of adjectives that end in -e in the singular and change to -i in the plural, regardless of whether the noun is masculine or feminine.

How do we know castelli is masculine?

The singular form is castello (castle). Nouns ending in -o in the singular are almost always masculine, and they change their ending to -i to become plural.

Why is the word for "these" questi instead of questo or queste?

Questo means "this", but just like other adjectives, it has to match the noun it points to! Since castelli is masculine and plural, questo changes to its masculine plural form, questi ("these"). If we were talking about feminine plural nouns, we would use queste.

Do adjectives after the verb sono still have to match the noun?