El perro negro es grande.
English: The black dog is big.
In Spanish, descriptive adjectives must match (agree with) the gender (masculine/feminine) and number (singular/plural) of the noun they describe.
Use this agreement whenever you use a descriptive adjective to describe a noun in Spanish. The adjective changes its ending to match whether the noun is masculine or feminine, and singular or plural.
El perro negro es grande.
English: The black dog is big.
La casa blanca es bonita.
English: The white house is pretty.
Los coches rápidos son nuevos.
English: The fast cars are new.
Las chicas simpáticas hablan mucho.
English: The nice girls talk a lot.