Indefinite pronouns name people, things, or amounts in an imprecise way. They do not point to who, what, or how much exactly, but leave the reference open: alguien, nada, muchos, varias. In the sentence they substitute a noun that one does not want or cannot identify. Without them, one would have to repeat the noun or give a longer explanation. With them, the sentence keeps its sense but the reference becomes general. They can appear alone as pronouns or accompany a noun as determiners, as happens with many Pronombres.

Some indefinidos change to agree with the noun or the person they represent. alguno and ninguno have masculine and feminine forms, singular and plural: alguno, alguna, algunos, algunas; ninguno, ninguna, ningunos, ningunas. todo also agrees: todo, toda, todos, todas. Others keep a single form, such as alguien, nadie, algo, nada, cualquier, quienquiera. When the indefinite functions as a determiner, the agreement appears in the noun it accompanies; when it functions as a pronoun, the form of the indefinite already expresses that relationship. Thus, alguna casa and alguna do not use the same form, although the agreement remains present.

Basic agreement of the indefinites

Idea
Some indefinite pronouns change form to agree with gender and number.
Other indefinite pronouns maintain a single form in both singular and plural.

Alguien is used for an unidentified person: Alguien llamó por teléfono. Nadie expresses the absence of a person: No hay nadie en casa. Algo refers to a thing, idea, or unprecised situation: Tengo algo para ti. Nada denies the existence of a thing or quantity: No encontré nada. These forms do not change in gender or number. Alguien and nadie are used with people; algo and nada, with things, facts, or ideas. In a sentence, they usually appear before or after the verb depending on the type of sentence, and they are common with Pronombres Relativos when the reference is specified later.

Basic usage of indefinite referents

Usage	Explanation
Unidentified person	Use someone when you know it is a person, but you don't know who.
Absence of person	Use nobody to say that there is no person in that place or situation.
Unspecified thing	Use something when you talk about an indeterminate or little concrete thing.
Absence of thing	Use nothing to say that there is no thing in the mentioned situation.

Alguno indicates an amount or existence that is not exact, usually among several possibilities: Algún día volveré; Tengo algunos libros. Ninguno expresses total absence: No tengo ninguna idea. Todo denotes totality: Todo el pan se acabó; Todos llegaron tarde. Alguno and ninguno change according to gender and number. Todo also agrees, and can refer to a single thing or to an entire set. When they come before a masculine singular noun, alguno and ninguno change to algún and ningún: algún problema, ningún amigo. That reduced form appears only before the noun.

Variable forms of quantity and inclusion

Idea
Some and none change gender and number when accompanying a reference to people or things.
All can vary to express totality in masculine or feminine, singular or plural.
None expresses complete absence of elements within the mentioned group.

A single indefinite can be a pronoun or a determiner. It is a pronoun when it substitutes the noun: No vi a ninguno; Compré algunos. It is a determiner when it accompanies the noun: ningún coche, algunos amigos. The difference depends on whether the noun appears or not. If the noun is present, the indefinite accompanies it; if it does not appear, the indefinite takes its place. With todo the same happens: todo el vino is determiner, and Todo se terminó with todo functions as a pronoun. This alternation is also seen in Pronombres Posesivos and Pronombres Demostrativos, where the presence of the noun changes the function of the word.

Pronominal or determiner value

Usage	Explanation
Substitution of the noun	Functions as a pronoun when it replaces the noun and does not accompany it.
Noun accompaniment	Functions as a determiner when it accompanies the noun and specifies it.
Reference without repetition	Use the pronoun to avoid repeating the name already known in the conversation.

Quienquiera is used to talk about an unidentified person, with a general and formal sense: Quienquiera que llegue primero, avise. Cualquiera indicates indifference among several people or things: Cualquiera puede hacerlo; Toma cualquiera de estos cuadernos. When it functions as a pronoun, it can stand alone or with a completing expression. It can also appear before the noun, as in cualquier día or canyuer persona, where it loses part of its pronoun strength and acts as a determiner. Cualquiera agrees in plural with cualesquiera, a less common and more formal form.

Non-specific people and indifference

Usage	Explanation
Indifferent person	Use anyone when it doesn't matter which person performs the action or receives the result.
Free or open value	Use whomever to refer to any person without choosing one in particular.
Generalizing tone	These forms serve to talk about non-specific people in very general situations.

Cada distributes the idea of one by one. It does not present a group as a block, but as a set of elements considered individually: Cada alumno tiene un libro. When reinforced with uno, cada uno appears, pointing to a person or thing separately: Cada uno eligió su asiento. Cada goes before a singular noun, because it does not refer to several at once but to each element separately. It is not used with the plural. Cada uno can function as a pronoun and substitute for the group that is already known in context. The distributive value also appears in expressions like cada día, cada vez or cada lado.

Individual distribution of elements

Usage	Explanation
Individual distribution	Use each one when you want to emphasize that every element is considered separately.
Emphasis on unity	Use this expression to mark that the whole group is not spoken of as a block.
Order or counting	Used to present a series when each member counts individually.

In negative sentences, nadie, nada and ninguno reinforce the negation. No vi a nadie, No tengo nada, No hay ningún problema. In Spanish, these words can appear next to no without the sentence being incorrect. When they go before the verb, they usually bear the negation in the verb: Nadie vino, Nada ocurrió. Algo changes value in a negative context and can mean 'nothing' or 'absolutely' in expressions like no algo, although in normal usage nada is preferred. The relationship between these forms and negation is also studied in Pronombres Dobles, where the same phrase can contain more than one pronoun with different functions.

Effect of negation on indefinites

Idea
In a negative sentence, nobody and nothing strengthen the negation.
After no, none expresses complete absence in the mentioned group.
Something can take a negative value when it appears in a context of double negation.

Some indefinidos shorten their form before a masculine singular noun. Alguno becomes algún: algún vecino. Ninguno becomes ningún: ningún coche. Cualquiera shortens to cualquier: cualquier persona. These clipped forms appear only before the noun and are not used when the indefinite stands alone. The choice depends on the position in the sentence, not on the meaning. There are also more literary or less common usage variants, such as quienquiera and cualesquiera, which maintain the same general value but with a more formal register. In quantity contexts, these forms can combine with Pronombres Numerales to express ideas like indefinite quantity, totality, or absence without specifying an exact number.

Reduced forms and common variants

Idea
Some loses its final vowel and becomes some before a masculine singular noun.
None loses its final vowel and becomes no before a masculine singular noun.
Anyone keeps its full form when it goes after the noun or as a pronoun.