How often, in English, do you use words like somebody, nobody, something, too or enough? They’re small words, but essential ones: they let you talk about people, objects or quantities without pinning them down exactly. Spanish has its own equivalents, called adjetivos and pronombres indefinidos.

In this article, we’ll look at what they are, how they work, and where the rules of agreement (and the traps that trip up English speakers in particular) lie, with practical examples taken from everyday Spanish.

1. What are the indefinidos?

The indefinidos are words that refer to people, things or quantities without identifying them precisely. In Spanish, they fall into two main groups:

• Invariable: they never change form, regardless of gender or number.
• Variable: they agree in gender and number with the noun they refer to.

Let’s go through them one by one, starting with the invariable ones, which are also the easiest to learn.

2. Indefinidos invariables: alguien, nadie, algo, nada

These four indefinites never change form: no gender agreement, no number agreement. And the verb that goes with them is always in the third person singular.

2.1. Alguien y nadie: talking about people

Alguien means “somebody/someone” and nadie means “nobody/no one”. They’re always pronouns: they never accompany a noun, they replace it, and you use them when you don’t know (or don’t want to specify) who you’re talking about.

– ¿Hay alguien en la oficina? / No, no hay nadie. Han ido a comer.
– Necesito alguien fuerte para llevar la maleta.

Notice one important thing: alguien and nadie can be followed by an adjective (like fuerte or interesante), and in this case the adjective is always masculine singular, because it doesn’t refer to a specific person but to a general idea.

Here’s the point where English speakers really need to be careful, though: English avoids double negatives. You’d say “There isn’t anybody” or “There’s nobody”, never “There isn’t nobody” (which would sound wrong, or would even flip the meaning back to a positive in careful English). Spanish, however, works the opposite way: when nadie or nada comes after the verb, the sentence still needs no before the verb. So No hay nadie is correct and expected — literally “There isn’t nobody” — and this “double negative” is not a mistake, it’s the rule. This is probably the single biggest reflex you’ll need to unlearn.

2.2. Algo y nada: talking about things

Algo means “something” and nada means “nothing”. These are also always pronouns, and they work exactly like alguien and nadie, but they refer to objects, ideas or concepts rather than people.

– ¿Quieres algo más? / No, gracias, ahora no quiero nada.
– Debo decirte algo increíble.

As with alguien and nadie, algo and nada can be followed by an adjective, always invariable and masculine singular (algo increíble, nada bonito).

3. Indefinidos variables: alguno, ninguno and their forms

Unlike the previous ones, alguno/a/os/as and ninguno/a/os/as agree with the noun they refer to, in both gender and number. This is genuinely new territory for an English speaker, since English adjectives and determiners never change form for gender or number (“some”, “any”, “no” stay exactly the same whether you’re talking about one book or five). These words are used for both people and things, and they can work in two ways:

• as adjectives, when they accompany a noun directly:

– Algunos alumnos han copiado el examen.
– Tengo cuatro hermanos y ninguna hermana.

• as pronouns, when they replace the noun, already mentioned earlier:

– ¿Tienes alguna camisa azul? / No, no tengo ninguna.

3.1. Watch out for the apocope: algún and ningún

Here’s a rule with no real equivalent in English, so it needs a bit of extra attention: alguno and ninguno drop the final -o and take a written accent when they come directly before a masculine singular noun. They become algún and ningún.

– ¿Tienes algún periódico de hoy? / No, no tengo ninguno.
– Ningún miembro del partido ha dado su voto todavía.

Note: this “apocope” (the dropping of the final vowel) happens only before a masculine singular noun. If the word stands alone (used as a pronoun, as in the reply no tengo ninguno), the -o doesn’t drop and the accent disappears.

4. Demasiado y bastante: talking about quantity

Now let’s move on to two words used constantly in everyday life: demasiado (too/too much) and bastante (enough/quite a lot). Here the rule shifts slightly from what we’ve seen so far, because these words can behave in two different ways.

4.1. As adverbs (invariable form)

When demasiado and bastante go with an adjective, an adverb, or come after the verb, they stay invariable: they don’t agree with anything, because in this case they function as adverbs — exactly like too and enough in English, which never change form.

– El café está demasiado caliente.
– Vivimos bastante lejos del centro.

4.2. As adjectives (variable form)

This is where things diverge from English. When demasiado and bastante accompany a noun directly, they become adjectives and agree in gender and number: demasiado, demasiada, demasiados, demasiadas and bastante, bastantes.

English doesn’t mark gender at all, and it handles this differently too: instead of changing the word’s ending, English switches between too much / too many and enough, depending on whether the noun is countable or not (“too much work” vs. “too many problems”). In Spanish, that same distinction is expressed through gender and number agreement rather than through a change of word.

– Debemos ordenar la habitación, hay demasiadas cosas por el suelo.
– No quiero hablar más, ya tengo bastantes problemas.

The trick to getting it right is simple: always ask yourself what the word relates to. If it’s followed by a noun, it agrees with it; if it goes with an adjective, an adverb, or stands alone after the verb, it stays invariable.

5. Why does it matter to master the indefinidos?

The indefinidos are among the most frequent words in spoken Spanish. You’ll come across them in everyday questions (¿hay alguien ahí?), in negative answers (no, no hay nada), in descriptions of quantity (tengo demasiado trabajo), and in countless practical situations: at a restaurant, at the office, on the street, in a casual conversation with friends.

Mastering them lets you:

• Ask questions and give natural answers without stumbling over your words.
• Avoid the mistakes English speakers make most often, like forgetting the accent on algún and ningún, dropping the no before the verb out of habit, or forgetting to make bastante and demasiado agree with a noun.
• Sound more natural and closer to a native speaker, because these indefinites come up in pretty much every conversation.

6. A quick recap

To help everything stick, here’s a fast summary:

• Invariable, for people: alguien, nadie (+ singular verb, + invariable adjective).
• Invariable, for things: algo, nada (+ singular verb, + invariable adjective).
• Variable, for people and things: alguno/a/os/as, ninguno/a (watch out for the apocope algún/ningún before a masculine singular noun).
• Demasiado/bastante: invariable when they go with an adjective, adverb or verb; variable when they accompany a noun directly.

7. Want to go deeper?

If you’d like to go through all these rules at your own pace, with clear, ready-to-use summaries while you study or do exercises, check out the grammar sheet on Los indefinidos in the Sillabi grammar resources. It’s the perfect tool for locking in the rules and always having a quick reference on hand.

8. And to keep learning Spanish in a structured way…

Theory is essential, but to actually use alguien, nadie, algo, nada, alguno, ninguno, demasiado and bastante naturally, you need consistent practice in real contexts. If you want to build a solid grammar foundation, speak with more confidence, and express yourself fluently from the first few months, check out the ¡Ojalá! course: a progressive, practical and motivating path designed to help you understand real Spanish, use it in everyday situations, and build fluency without the stress.

And if you’re not sure where to start, take the level test: in just a few minutes, it will point you to the best place in the course to begin.