In Spanish grammar, adjectives have to agree with the nouns they modify in both gender and number, no matter what:
Gender: If a noun is feminine, like la muchacha (the girl), the adjective must be feminine, too. For example, to talk about a tall girl, you’d say la muchacha alta (the tall girl). If the girl has a brother who’s also tall, you’d say el muchacho alto (the tall boy).
Number: If a noun is plural, the adjective must also be plural. For example, to describe a group of tall girls, you’d say las muchachas altas. To describe a group of tall boys, you’d say los muchachos altos. Similarly, if a noun is singular, the adjective must be singular, too (see the preceding bullet for examples).
Following are some general rules about making adjectives agree with the nouns they modify:
Like nouns, most adjectives follow the general rule that masculine adjectives end in -o and pluralize with -s and feminine adjectives end in -a and pluralize with -s.
Adjectives that end in a consonant, -e, or -ista usually don’t have masculine and feminine forms, but they do have singular and plural forms. To make an adjective that ends in -e or -ista plural, simply add -s. To make an adjective that ends in a consonant plural, add -es.
With some adjectives that end in -dor, -ón, or -án, you add -a to form the feminine, -es to form the masculine plural, and -as to form the feminine plural.
Here are a few more examples of adjectives that agree with the nouns they modify in both gender and number:
un examen difícil (a difficult exam)
una chica inteligente (a smart girl)
unos peces caros (some expensive fish)
unas reglas importantes (some important rules)
