Adjectives are the words we use to describe the nouns. They can tell you any characteristic of the noun they go with. This could be about its shape, colour, size, etc. In Spanish, adjectives are variable words, that is, they are going to take on different forms depending on the noun they go with, as they always have to agree with it. If the noun is singular, then the adjective will be singular too and if the noun is plural then the adjective will also be plural. You need to learn that in Spanish the nouns possess gender and that many adjectives have gender as well.

la casa bonita - the beautiful house
las casas bonitas - the beautiful houses

el carro rojo - the red car
los carros rojos - the red cars

Adjectives in Spanish usually go after the noun.

Compraron un carro (noun) nuevo (adjective). - They bought a new car.

In Spanish, masculine adjectives normally end in "-o" and the ones ending in "-a" are normally feminine. These adjectives are going to take on four different forms each, depending upon the noun they describe. So, if we take as an example the adjective "bonito" (beautiful) we will find it in four different forms depending on the noun it goes with:

el libro bonito - the beautiful book (masculine, singular)

los libros bonitos - the beautiful books (masculine, plural)

la chica bonita - the beautiful girl (feminine, singular)

las chicas bonitas - the beautiful girls (feminine, plural)

As you can see from the examples above, the endings of the nouns and the adjectives that go with them are the same; that is because the adjective has the same gender and number as the noun it is describing. There are other adjectives in Spanish that only change their form to distinguish between singular and plural but not between masculine and feminine. These adjectives end in "-e".

una persona importante - an important person
unas personas importantes - some important people

un hombre importante - an important man
unos hombres importantes - some important men

We have studied that adjectives take on four different forms: masculine, feminine, singular and plural. We are now going to learn the rules to form the plural forms. Normally we just add "-s" to the singular form. Eg:

grande - big
grandes - big

If the adjective in its singular form ends in a stressed vowel or consonant, we will add "-es" to form the plural.

marrón - brown
marrones - brown

Israelí - Israeli
Israelíes - Israelis

If the adjective ends in "-z" the plural form ends in "-ces" Eg:

feliz - happy
felices - happy