The definite article indicates that the noun is known, concrete, or already mentioned. In el libro, la mesa, los vecinos and las llaves, the word does not present a new thing, but one that the speaker and the listener can identify. If you say vi un libro, you are talking about any book; if you say vi el libro, you are talking about one in particular. It also appears when the context has already made it clear: Compré un pan. El pan estaba caliente. Its function is to add precision and turn the noun into something determinate.

Agreement of the forms of the definite article

Idea
The definite article must agree with the gender and number of the noun.
The form la accompanies feminine singular nouns.
The form los accompanies masculine plural nouns.
The form las accompanies feminine plural nouns.

The forms are el, la, los, and las. El accompanies masculine singular nouns, la accompanies feminine singular nouns, los accompanies masculine plural nouns, and las accompanies feminine plural nouns. The article agrees with the noun in gender and number, never with the adjective. We say el coche rojo, la casa roja, los coches rojos and las casas rojas. When the noun changes number, the article changes with it. To compare with other forms of determination, it is useful to review Artículos indefinidos, where the noun is presented as non-specific.

Use of the article in singular

Usage	Explanation
Masculine singular noun	Use el in front of a masculine singular noun to indicate something known or concrete.
Feminine singular noun	Use la in front of a feminine singular noun to present a specific referent.

With masculine singular nouns, el is used: el bolso, el profesor, el problema. With feminine singular nouns, la is used: la ventana, la ciudad, la idea. The article goes in front of the noun and also in front of the adjective when the adjective is part of the noun phrase: el coche nuevo, la ropa limpia. If a feminine noun begins with a stressed a, the form changes to el in singular: el agua fría, el hacha vieja, el aula pequeña. The gender remains feminine; only the article form changes for pronunciation.

Use of the article in plural

Usage	Explanation
Masculine plural noun	Use los in front of a masculine plural noun to talk about a definite group.
Feminine plural noun	Use las in front of a feminine plural noun to name several known or specific things.

With masculine plural nouns, los is used: los libros, los amigos, los problemas. With feminine plural nouns, las is used: las sillas, las calles, las respuestas. The agreement is maintained even if the noun carries adjectives or complements: los platos limpios, las casas del barrio. When the singular is el due to sound clash, the plural returns to the normal feminine form: el agua, but las aguas. The same happens with el aula and las aulas.

Article before initial stressed vowel

Idea
Use el with some feminine nouns that start with a stressed a to avoid sound clash.
The singular form becomes feminine again in adjectives and in agreement.
The exception remains with words like aula and hacha when the first syllable sounds strong.

In front of a feminine singular noun that begins with a stressed a or ha, el is used to avoid the clash of sounds: el agua, el aula, el hacha, el hambre. The noun remains feminine, so the adjectives and determiners that accompany it keep feminine agreement: el agua fría, el aula grande, el hacha nueva. In plural it becomes las: las aguas, las aulas, las hachas. This change in form does not occur with other words that do not have that initial pronunciation.

el artículo ante nombres de algunos ríos y maresel artículo ante algunos nombres de países y montañasel artículo con nombres de idiomas en expresiones concretas

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Se usa el artículo con ciertos nombres geográficos para señalar el lugar de forma tradicional o fijaEn usos específicos, el artículo aparece con nombres de idiomas cuando se habla del idioma como materia o asuntoEn ciertos contextos, el artículo acompaña nombres propios para darles un tono más natural o tradicional

Use of the article with proper names and places

Region	Variant	Definition	Example
Spain	el artículo ante nombres de algunos ríos y mares	The article is used with certain geographic names to signal the place in a traditional or fixed way.	Navegamos por el Amazonas con un pato guía.We sail along the Amazon with a guide duck.
Mexico	el artículo ante algunos nombres de países y montañas	In certain contexts, the article accompanies proper names to give them a more natural or traditional tone.	Subimos el Popocatépetl con chocolate caliente.We climb the Popocatépetl with hot chocolate.
Argentina	el artículo con nombres de idiomas en expresiones concretas	In specific uses, the article appears with names of languages when talking about the language as a subject.	Estudia el español con una vaquita detective.Studies the Spanish with a detective cow.

Scramble

With proper names of people the article appears when the speaker wants to give familiarity, emphasis, or concrete reference: la María de la que te hablé, el Juan del quinto. With surnames in plural it is also normal: los García, las Gómez. In geographic names, usage depends on the name. Many countries do not take an article, but some do: la Argentina, el Perú in certain contexts, los Países Bajos. It is also used with rivers, seas, oceans, mountain ranges and mountains: el Amazonas, el Mediterráneo, los Andes. With languages, the article appears especially when the language functions as a noun: el español es una lengua flexible, as also seen in Omisión del Artículo and in broader uses related to Adjetivos Demostrativos.

Use of the article with days of the week

Usage	Explanation
Weekly habit	Use el with a day of the week when you talk about a habit or repetition.
General reference	Use el with a day to mention that moment in general, not as a concrete date.

With days of the week, the article is used to talk about habits, repetitions, or a general reference: El lunes trabajo temprano, Los martes estudio francés, El domingo descansamos. Here the day does not point to a specific date, but a type of day within a routine or a general idea. If you speak of a specific date, normally you do not need the article: Lunes voy al médico appears in some very brief styles, but in standard Spanish the full form for a general reference is with the article. It is also used in expressions like los viernes por la noche and el día de la fiesta.

Article with implicit possessive

Usage	Explanation
Part of the body affected	Use the article when the possession relationship is clear from context and no explicit possessive is needed.
Garment worn	Use the article when the garment is understood as part of the person and the possessive is implicit.

When the possessive is implicit, the article may appear with parts of the body and items of clothing: Me lavé las manos, Le duele la cabeza, Se puso el abrigo. The possession relationship is understood from context and the verb, not from an explicit possessive. Against mi mano or su abrigo, the article presents the body part or garment as linked to the subject of the sentence. This is very common with action verbs on the body itself and with verbs of changing clothes: Se quitó los zapatos, Nos tocamos la frente, Abrió los ojos. If the possessive is stated explicitly, the article usually disappears: mi mano, tu chaqueta.

Omission of the article in fixed uses

Idea
In some headings and labels, the article is omitted to give a brief and clear style.
In lists and titles, the article is omitted when the phrase functions as a label or name.
In certain fixed expressions, the article does not appear because the phrase is already established this way in usage.

The article is omitted in titles, signs, headers, lists, notices, and section names: Entrada, Salida, Capítulo dos, Panadería, Prohibido fumar. It is also missing in many fixed expressions and short formulas: ir a casa, tener hambre, dar clase, hacer falta. In those cases, the noun functions in a stable way and does not require a determinate reference. Instead, when one wants to indicate something concrete, the article returns: la entrada del museo, la clase de hoy, la casa de mi hermana. To see more contrasts of presence and absence of the article, it is useful to compare with Omisión del Artículo and with Artículo neutro lo, which serves a different function.