This document discusses the rules for adjective agreement in Spanish. It explains that adjectives must match the gender and number of the nouns they describe. For gender, masculine adjectives typically end in "-o" while feminine adjectives end in "-a". However, some adjectives ending in "-or" add "-a" for the feminine. Most adjectives ending in "-e" or consonants are the same for both genders. For number, adjectives made plural by adding "-s" if they end in a vowel and "-es" if they end in a consonant. When adjectives describe mixed gender groups, the masculine form is used.

1. Las reglas de concordancia para los adjetivos
Español I
Using adjectives to describe GENDER
Adjectives describe nouns. Like articles, they must match the gender of the
noun they describe. In Spanish, adjectives usually follow the noun.
Masculine adjectives often Feminine adjectives often
end in “-o”: end in “-a”:
el chico guapo la chica guapa
For some adjectives (e.g., those that end in “-or”), you must add an “-a” to
make the feminine form:
el alumno trabajador la alumna trabajadora
Most adjectives that end in “-e” match both genders:
el profesor inteligente la profesora inteligente
Most adjectives that end in a consonant match both genders:
un examen difícil una clase difícil
Using adjectives to describe NUMBER

2. Adjectives must also match the number of the nouns that they describe. To
make an adjective plural, add “-s” if it ends with a vowel; if it ends with a
consonant, add “-es”.
los chicos guapos, divertidos y las chicas guapas, divertidas y
populares populares
When an adjective describes a group with both genders, the masculine
form of the adjective is used:
El chico y la chica son guapos.