Adjectives must agree with the nouns they modify. When you learn a new adjective as a vocabulary word, you'll see it presented in the masculine/singular form. Additionally, most adjectives have a plural form, and many have feminine/singular and feminine/plural endings as well.

Frequently, an adjective's masculine/singular form will end in -o. If such is the case, its three other forms are -a, -os, and -as. Take a look at the adjective rojo as an example:

Almost all other adjectives end with a consonant or -e. These adjectives generally don't change to reflect gender — that is, they only have two forms: singular and plural. The plural form is constructed by adding -es to adjectives that end in consonant and -s to adjectives that end in -e:

Don't forget that adding -es in the plural may necessitate a change in the use of accent marks or a spelling modification. One common change occurs with adjectives that end -z. Because sounds “ze” and “zi” almost never occur in Spanish, the spelling is modified to -ces to reflect correct pronunciation: