Indefinite adjectives accompany the noun to indicate a quantity, an identity, or an imprecise reference. They say we are talking about a part, a class, or an unspecified element within the noun phrase. In vi a varios estudiantes, varios does not identify who they are; it only marks that there is more than one. In necesito cierta ayuda, cierta leaves the noun in a vague reference. If the adjective is removed, the noun still exists, but loses that information of quantity or precision. Compared to the Descriptive Adjectives, which add a quality, the indefinites place the noun in a measure, a selection, or an imprecise reference.

Agreement with the noun

Idea
The indefinite adjective accompanies the noun and adapts to its gender and number when the form allows.
If the noun changes, the form of the variable indefinite adjective also changes.

An indefinite adjective accompanies the noun and, when its form allows it, agrees with it in gender and number. That agreement follows the general rules of Concordancia Adjetival. One says muchos libros, muchas mesas, algunas casas, otros días. When the indefinite has a single form, it does not change even if the noun is masculine or feminine, singular or plural: cada día, cada persona. The usual position is in front of the noun: varios problemas, poco tiempo, tres amigos. Some indefinites can also go after in more marked uses, but the anteposed placement is the most common in general-use Spanish.

Forms that change and fixed forms

Idea
Indefinite adjectives that are variable accept agreement with the noun in gender and number.
Indefinite adjectives that are invariable keep the same form even if the noun changes.
Sendos expresses distribution and always appears in plural.

There are indefinite adjectives that change form and others that do not. Mucho, poco, otro, varios and tantos are variable: the noun dictates the gender and number, so they appear as mucho ruido, mucha agua, muchos libros, muchas ganas; otro amigo, otra amiga, otros amigos, otras amigas; varias cartas; tantos días. By contrast, cada, cualquier and sendos do not show gender variation, and their behavior is fixed. Cada always goes with a singular noun: cada persona, cada decisión. Cualquier tends to stay the same and expresses non-precise choice: cualquier día, cualquier solución. Sendos does change in number, but not in gender within each number: sendos regalos, sendas cartas. These adjectives are used before the noun and form with it a single noun phrase.

Use of singular and apocope

Idea
Some and none are used in front of a singular noun.
In front of a masculine singular noun, alguno becomes algún.
In front of a masculine singular noun, ninguno becomes ningún.

Algún y ningún go before a singular noun. Algún introduces an imprecise reference: algún problema, alguna idea. Ningún expresses total absence: ningún motivo, ninguna respuesta. In front of a masculine singular noun, the two forms become clumped together: algún libro, ningún amigo. The full form appears before feminine nouns: alguna casa, ninguna opción. With plurals they are not used as preposed adjectives in the same way; in that case other constructions are used. Their normal place is before the noun, and that position retains its indefiniteness or negation value.

Non-specific choice with cualquiera

Usage	Explanation
Before a noun	Use cualquiera when the option is indifferent or not specific and may change form before the noun.
Free reference	Use cualquiera to talk about a person or thing without pointing to a specific one.

Cualquiera se emplea para hablar de una opción no precisa, entre varias posibles. Antepuesto al sustantivo, muestra que no se elige una persona, cosa o momento concreto: cualquier día, cualquier persona, cualquier excusa. When it goes before a masculine singular noun, it loses the -a ending and appears as cualquier: cualquier libro. With feminine singular nouns, the full form is maintained: cualquiera de las dos rutas, cualquiera noche no es el uso normal ante sustantivo; la forma habitual es cualquier noche. También puede funcionar como pronombre, pero como adjetivo indefinido siempre acompaña al sustantivo y señala indiferencia o falta de precisión en la elección.

Comparative quantity with tanto

Usage	Explanation
Quantity comparison	Use tanto to express a large amount in comparison to another and it agrees with the noun.
With a feminine noun	Use tanta when the noun is feminine and the amount is presented as high.

Tanto expresses comparative quantity and agrees with the noun it accompanies. It indicates a large or equal quantity in relation to another: tanto trabajo, tanta paciencia, tantos errores, tantas preguntas. It agrees in gender and number with the noun, just like other variable adjectives. Its value changes depending on the context: it can mark high quantity, equivalence or intensity in a comparison. In no tengo tanto tiempo, the quantity is large but not exact; in hay tantas sillas como mesas, a comparative relationship appears. It also enters structures with Adjetivos Cuantitativos, because it expresses measure, but here it functions as an indefinite adjective when it directly accompanies the noun.

Approximation and vague reference with cierto

Usage	Explanation
Elegant approximation	Use cierto to indicate an approximate reference or nuance.
Formal or ambiguous tone	Use cierto when you want to sound more formal or leave the reference somewhat fuzzy.

Cierto presents a noun as known in a vague, approximate or not entirely precise way. In cierto miedo, cierta distancia or ciertos comentarios, the speaker does not want or cannot identify the element exactly. It agrees with the noun: cierto rumor, cierta idea, ciertos vecinos, ciertas dudas. In real speech it can sound more formal than other indefinidos and sometimes leaves an ambiguous nuance, because it suggests a recognizable reference without naming it. In that use, it functions as a frame of approximation, not as a marker of precision. For that reason it pairs well with nouns that admit a general reference, as in cierta tensión en la sala or cierta urgencia en el mensaje.