The singular names a single person, animal, thing, or idea: book, table, girl. The plural names several: books, tables, girls. The number appears in the noun and then repeats in the words that accompany it, as in Definite Articles, Indefinite Articles and adjectives. If I say la casa, everything is singular; if I say las casas, everything is plural. The singular is used to identify a concrete element or to talk about an idea as a unit. The plural is used to count more than one and to talk about complete groups.

Basic use of singular and plural

Idea
Use the singular to talk about a single person, animal, or thing.
Use the plural to talk about several people, animals, or things.
The nominal number helps to count and identify clearly.
The noun changes form when it goes from singular to plural.

Articles mark number and help present the noun. El and la accompany a known or identified singular noun; los and las accompany a plural noun. Un and una present a singular noun without identification; unos and unas do the same in plural. For example: el perro, la casa, los perros, las casas; un perro, una casa, unos perros, unas casas. The article agrees with the noun in gender and number, as in Género Nominal. When the noun changes to plural, the article changes as well: una amiga becomes unas amigas, never una amigas.

Selection of articles according to quantity and definition

Usage	Explanation
Known reference	Use the definite article when you talk about something that is already known or that you want to indicate with precision.
New reference	Use the indefinite article when you present something for the first time or without identifying it completely.
Known plural	Use the definite plural articles to talk about a group already identified.
New plural	Use the indefinite plural articles to present several elements without specifying them.

Negation uses ningún, ninguna, ningunos and ningunas to express total absence. Ningún and ninguna are used with singular nouns: ningún problema, ninguna respuesta. Ningunos and ningunas are used with plural nouns: ningunos zapatos, ningunas noticias. These forms take the place of an article and deny the existence of the noun in that situation. That is why they say there is not a single thing or that there are not several. In a sentence like No tengo ningunas llaves, the plural of the noun requires the plural of the determiner.

Use of negative articles with absence

Usage	Explanation
Negation of masculine singular	Use ningún to deny the existence of a masculine singular noun.
Negation of feminine singular	Use ninguna to deny the existence of a feminine singular noun.
Negation of masculine plural	Use ningunos to deny the existence of a masculine plural noun.
Negation of feminine plural	Use ningunas to deny the existence of a feminine plural noun.

Some words substitute the noun and keep its number. Uno and una replace a singular noun: Quiero una, where the context says which thing is being avoided from repeating. Algunos and algunas replace a group: Compré algunos or Vi algunas. Ninguno and ninguna deny the presence of a referent: No tengo ninguna. These forms are used when the noun is already understood from the context and there is no need to repeat it. The pronoun maintains number agreement with the idea it represents, just as with articles and adjectives.

Subject	Infinitive	Conjugation
singular masculino	uno
singular femenino negativo	ninguna
plural masculino	algunos
singular masculino negativo	ninguno
singular femenino	una
plural femenino	algunas

Pronouns that agree in number

Subject	Infinitive	Conjugation
singular masculino	uno	uno
singular femenino	una	una
plural masculino	algunos	algunos
plural femenino	algunas	algunas
singular masculino negativo	ninguno	ninguno
singular femenino negativo	ninguna	ninguna

Scramble

The adjective changes to agree with the noun in number. Casa grande becomes casas grandes; libro interesante becomes libros interesantes. If the noun is singular, the adjective is singular. If the noun is plural, the adjective is plural. This agreement also appears with adjectives that come after countable nouns, as in Countable Nouns. An adjective with a single ending keeps the same form in singular and plural only when that form already ends in -s or -x and the word does not change. In most cases, the visible change is -s in plural: alto + -s = altos, alta + -s = altas.

Agreement of the adjective with the noun

Idea
The adjective must agree in gender with the noun.
The adjective must agree in number with the noun.
If the noun changes to plural, the adjective also changes.
With several nouns, the adjective usually goes in plural.

The most common plural form adds -s to words ending in a vowel: casa > casas, niño > niños, cafés > cafés when the word already carries an accent for another reason. Words ending in a consonant form the plural with -es: hotel > hoteles, papel > papeles, mujer > mujeres. If the word ends in -z, the z changes to c and -es is added: luz > luces, voz > voces. These rules also apply to many forms that accompany the noun, such as articles and adjectives, to maintain agreement with Numeral Adjectives when expressing quantity.

General rules for forming plurals

Idea
Add s to words that end in a stressed vowel.
Add es to words that end in a consonant.
Words that end in z change to ces.
Words that end in a stressed vowel usually add es.

Some words do not follow the general rule. Nouns and adjectives that end in a stressed vowel typically form the plural with -s: sofá > sofás, mamá > mamás, café > cafés. Words ending in -y change that letter to -i and add -es: rey > reyes, ley > leyes. Other words do not change in plural, such as el lunes and los lunes, el tórax and los tórax. There are also nouns that are almost always used in plural, such as tijeras, gafas, and vacaciones, and are not normally used in singular. By contrast, some proper nouns and fixed terms keep a single form, even when they appear with or without an article, as happens with many Proper Nouns.

el régimenel tóraxel pieel caféel lápizel cumpleañosel lunesla crisislas tijeraslas gafas

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Su plural no sigue la forma regular y cambia la vocal internaHay sustantivos invariables que no cambian en pluralSu plural cambia de forma porque la palabra lleva acento al finalAlgunos sustantivos se usan casi siempre en pluralAlgunas palabras mantienen la misma forma en pluralCiertos nombres científicos o técnicos pueden ser invariablesAlgunas palabras mantienen la misma forma en singular y pluralSu plural usa una terminación especial por acabar en zHay sustantivos que se usan casi siempre en pluralAlgunos nombres de días pueden usarse igual en singular y plural

Irregular plurals and special uses

Word	Definition
el café	Its plural changes form because the word carries an accent at the end.
el pie	Its plural does not follow the regular form and changes the internal vowel.
el lápiz	Its plural uses a special ending because it ends in z.
el régimen	Some words keep the same form in plural.
las gafas	Some nouns are almost always used in plural.
las tijeras	There are nouns that are almost always used in plural.
el cumpleaños	Some words keep the same form in singular and plural.
la crisis	There are invariable nouns that do not change in plural.
el tórax	Certain scientific or technical names can be invariable.
el lunes	Some day names can be used the same in singular and plural.

Scramble

The courtesy plural uses ustedes to address a single person with respect or social distance, or multiple people. In modern Spanish, usted and ustedes replace tú and vosotros in many regions, and the verb always takes the plural form: ¿Cómo están ustedes?, ¿Quieren pasar?. When referring to a single person with great respect, the plural remains in the verb and in the pronouns that accompany the form of address. The grammatical number here does not describe real quantity, but the way of addressing someone.

Plural used out of respect or courtesy

Usage	Explanation
Respectful treatment	Use ustedes to address one person or several people with respect.
Institutional respect	The plural can sound more courteous when talking to an authority or a client.
Ceremonious style	Sometimes the plural gives a more formal and friendly tone.