In Spanish as in English, most of the words used as indefinite pronouns sometimes function as other parts of speech, often as adjectives and sometimes as adverbs. In Spanish, some of the indefinite pronouns exist in both masculine and feminine forms as well as singular and plural forms, so they must agree with the nouns they refer to.
Here are the indefinite pronouns of Spanish with examples of their use:
alguien — someone, somebody, anyone, anybody — Necesito a alguien que pueda escribir. (I need someone who can write.) ¿Me llamó alguien? (Did anybody call me?)
algo — something — Veo algo grande y blanco. (I see something big and white.) ¿Aprendiste algo esta tarde? (Did you learn something this afternoon?)
alguno, alguna, algunos, algunas — one, some (things or people) — Puedes suscribirte a alguno de nuestros servicios. (You can subscribe to one of our services.) ¿Quieres alguno más? (Do you want some more?) Voy a estudiar con algunas de las madres. (I'm going to study with some of the mothers.) Algunos quieren salir. (Some want to leave.)
cualquiera — anybody, anyone — Cualquiera puede tocar la guitarra. (Anyone can play the guitar.) — The plural form, cualesquiera, is seldom used.
mucho, mucha, muchos, muchas — much, many — Me queda mucho por hacer. (I have much left to do.) La escuela tiene mucho que ofrecer. (The school has much to offer.) Somos muchos. (There are many of us. Literally, we are many.)
nada — nothing — Nada me parece cierto. (Nothing seems certain to me.) No tengo nada. (I have nothing.) — Note that when nada follows a verb, the part of the sentence preceding the verb typically is also put in negative form, making a double negative.
nadie — nobody, no one — Nadie me cree. (Nobody believes me.) No conozco a nadie. (I know nobody.) — Note that when nadie follows a verb, the part of the sentence preceding the verb typically is also put in negative form, making a double negative.
ninguno, ninguna — none, nobody, no one — Ninguna de ellas va al parque. (None of them are going to the park.) No conozco a ninguno. (I know nobody. — Note that when ninguno follows a verb, the part of the sentence preceding the verb typically is also put in negative form. Plural forms (ningunos and ningunas) exist but are seldom used.
otro, otra, otros, otras — another, other one, another one, other ones, others — Quiero otro. (I want another one.) Los otros van al parque. (The others are going to the park.) — Un otro and una otra are not used for "another one." Otros and the related pronouns can be combined with a definite article (el, la, los or las) as in the second example.
poco, poca, pocos, pocas — little, little bit, few, a few — Tengo un poco de miedo. (I have a little bit of fear.) Pocos van al parque. (A few are going to the park.)
todo, todos, todas — everything, all, everyone — Èl comió todo. (He ate everything.) Todos van al parque. (All are going to the park.) — In singular form, todo exists only in the neuter (todo).
uno, una, unos, unas — one, some — Uno no puede creer sin hacer. (One cannot believe without doing.) Unos quieren ganar más. (Some want to earn more.) Comí uno y deseché el otro. (I ate one and threw away the other.) — Uno and its variations are often used in conjunction with forms of otro, as in the third example.
Although some different pronouns are translated the same into English, they aren't necessarily interchangeable. Explaining some of the subtle differences in usage is beyond the scope of this lesson. In many cases, the pronouns can be translated in more than one way into English; you must rely on context in those cases to convey the meaning.