Spanish adjectives must agree in gender and number with the nouns they describe so that they can accurately and precisely describe the noun or pronoun in question. What does “agree” mean exactly? When the noun or pronoun is masculine/feminine, the Spanish adjective describing it must also be masculine/feminine. Likewise, when the noun or pronoun is singular/plural, its verb and any adjectives describing it must also be singular/plural.
There are two basic rules to follow to form the plural of adjectives in Spanish:
Add -s to singular adjectives ending in a vowel. For example, alto (tall) becomes altos, and interesante (interesting) becomes interesantes.
Add -es to singular adjectives ending in a consonant. For example, f‡cil (easy) turns into f‡ciles, and trabajador (hard-working) turns into trabajadores.
Just like with some nouns and pronouns, when speaking about mixed company (males and females, with no mind to number) make sure to use the masculine form of the adjective. Therefore, to state My sister and my brothers are blond in Spanish, you’d say Mi hermana y mis hermanos son rubios.
You should know that some singular Spanish adjectives don’t follow the basic rules for making plurals. Following are the guidelines for these exceptions:
Singular adjectives ending in -z change -z to -c in the plural and then add the ending. Feliz (happy) thus becomes felices.
Some adjectives add or drop an accent mark to maintain original stress. Inglés (English) becomes ingleses.