The adjective accompanies the noun and takes its gender and number. If the noun is masculine singular, the adjective is also in masculine singular: niño alto. If the noun is feminine plural, the adjective changes to feminine plural: casas blancas. When removing the adjective, the sentence still has basic sense, but loses the information of quality, quantity or trait that the adjective provides. Therefore a fixed form is not used for all nouns, but a form that matches the noun.

Basic Rules of Agreement

Idea
The adjective agrees with the gender of the noun it accompanies.
The adjective also agrees with the number of the noun.

Many adjectives ending in -o form the feminine with -a: alto and alta, bonito and bonita, nuevo and nueva. The stem stays the same and only the ending changes. In plural, that same alternation continues: altos, altas. It is used with nouns such as el chico alto, la casa alta, los chicos altos and las casas altas. The agreement marks which noun the adjective describes.

Gender Change in Adjectives ending in -o

Idea
Many adjectives ending in o change to a in feminine.
The root is preserved when the adjective changes gender.

Adjectives ending in -e usually serve for masculine and feminine without changing form: estudiante inteligente, hombre inteligente, mujer inteligente. Also many adjectives ending in a consonant function the same in both genders: fácil, azul, feliz. In these cases, the change appears mainly in number: inteligente and inteligentes, fácil and fáciles. The form does not distinguish gender, but it must agree in plural when the noun requires it.

Stable Endings and Simple Inflection

Idea
The adjectives ending in e tend to be used for both genders.
Many adjectives ending in a consonant only change in plural.

To form the plural, adjectives ending in an unstressed vowel add -s: claro → claros, rápida → rápidas. Those ending in a consonant add -es: feliz → felices, fácil → fáciles, andaluz → andaluces. If the adjective ends in -z, the z changes to c before -es. The plural ending must agree with the plural noun it accompanies: chicos felices, calles estrechas.

Plurals of Adjectives by Ending

Idea
The adjectives that end in a vowel usually form the plural by adding s.
The adjectives that end in a consonant usually form the plural by adding es.

The agreement is maintained even if the adjective goes before or after the noun. You say una gran casa and also una casa grande; in both cases the adjective agrees with casa. With ser and estar, the adjective follows the same rule: la sopa está fría, las paredes son blancas. The position changes the order, not the agreement. In the sentence, the adjective always adopts the form of the noun it modifies.

Position and Verb in Agreement

Usage	Explanation
Adjective before the noun	The agreement stays the same even if the adjective comes before the noun.
With the verbs ser and estar	With ser and estar, the adjective also must agree with the subject.

When the adjective accompanies several nouns of different gender, the usual form is masculine plural: el padre y la madre están contentos, la mesa y el sofá son nuevos. That masculine plural functions as a group form. Some adjectives do not change for gender or number because their form is fixed, such as gris, gratis or a pie. Others retain the same form due to frequent use in certain contexts. In those cases, the agreement does not alter the word, but the meaning remains linked to the noun it describes.

Adjetivo invariableMasculino plural

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Cuando hay sustantivos de distinto género, se usa el masculino plural como forma neutralAlgunos adjetivos no cambian de forma y se usan igual en singular y plural

Fixed use of agreement and mixed plurality

Region	Variant	Definition	Example
Español general	Masculino plural	When there are nouns of different genders, the masculine plural is used as a neutral form.	María y Pablo son amigos muy divertidos.María and Pablo are amigos very funny.
Español general	Adjetivo invariable	Some adjectives do not change form and are used the same in singular and plural.	La chaqueta es gratis y las chaquetas son gratis.The jacket is gratis and the jackets are gratis.

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