In short

Indefinites are the words that say how much, how many, and which unspecified one - algum livro (“some book”), muita gente (“a lot of people”), nada (“nothing”). Portuguese splits them into two families you must keep apart. Most are adjective-like and agree with their noun in gender and number, exactly like the adjectives of Lesson 5: muito -> muita -> muitos -> muitas. A handful are invariable neuter pronouns that stand alone and never change: tudo, nada, algo, alguém, ninguém. And the single biggest thing for an English speaker: Portuguese is a double-negative language. “I don’t want anything” is Não quero nada - literally “I don’t want nothing” - and that stacked negation is correct, obligatory, and everywhere.

The rule

Two families

1. Variable (agree in gender and number): algum, nenhum, todo, muito, pouco, tanto, outro, certo, vário(s). These sit before a noun like adjectives (alguns dias, muitas casas) and can also stand alone as pronouns (Quero outra).
2. Invariable neuter pronouns (one fixed form, no noun attached): tudo (everything), nada (nothing), algo (something), alguém (someone), ninguém (nobody). They point at unspecified people or things in the abstract, and they are always grammatically third-person singular.

Agreement, and the muito trap

A variable indefinite copies its noun’s gender and number: muito tempo, muita água, muitos amigos, muitas amigas. But the very same word muito / pouco is also an adverb, and as an adverb it is frozen - it never agrees. So when muito quantifies a noun it agrees (muitos erros), but when it modifies an adjective or a verb it does not: ela é muito simpática (not muita simpática), elas trabalham muito. Getting this one distinction right removes the most common A2 slip in the whole topic.

The affirmative/negative pairs and where não goes

Each affirmative indefinite has a negative partner: algum <-> nenhum, alguém <-> ninguém, algo / alguma coisa <-> nada, sempre <-> nunca, também <-> também não / nem. The placement of the negative word decides whether you also need não:

• Negative word after the verb -> keep não before the verb: Não conheço ninguém. / Não tenho nenhum. / Não há nada.
• Negative word before the verb -> drop não (one negation is enough): Ninguém veio. / Nunca fumei. / Nada mudou.

Both patterns are single grammatical negatives in Portuguese; the “double” you see in Não quero nada is required, not a mistake to be corrected out.

todo vs tudo

todo is the variable adjective “all / every / whole” and agrees; tudo is the invariable pronoun “everything.” English “all/everything” collapses the two, so learners mix them. Also keep these apart:

• todo o dia = “the whole day”; todos os dias = “every day.” Singular noun with article = whole; plural noun with article = every.
• EP says toda a gente for “everybody” (Brazilian Portuguese prefers todo mundo).

cada, qualquer, ambos

• cada (“each”) is invariable and always takes a singular noun: cada dia, cada pessoa; “each one” is cada um / cada uma.
• qualquer (“any / whichever”) has the irregular plural quaisquer: qualquer pergunta -> quaisquer perguntas.
• ambos os / ambas as = “both”: ambos os irmãos, ambas as mãos.

algo vs alguma coisa

Both mean “something.” In European Portuguese alguma coisa is the everyday spoken choice (Queres alguma coisa?), while algo is slightly more formal or written. The negative of both is nada.

Forms & patterns

Variable indefinites - the four agreement forms:

nenhum is almost always used in the singular; the plurals nenhuns / nenhumas exist but are rare.

Invariable words:

Affirmative and negative partners:

Examples

Common mistakes

• Quero nada. -> Não quero nada. - when the negative word follows the verb, EP keeps não; the double negative is obligatory, not an error to remove.
• Ela é muita simpática. -> Ela é muito simpática. - before an adjective muito is an adverb and stays invariable; it agrees only when it quantifies a noun (muitas casas).
• Comi todo. -> Comi tudo. - “everything” is the invariable pronoun tudo; todo is the agreeing adjective (todo o bolo = the whole cake).
• Todos os dia / cada dias -> todos os dias / cada dia - todos takes a plural noun, cada always a singular one.
• qualquer perguntas -> quaisquer perguntas (or qualquer pergunta) - the plural of qualquer is the irregular quaisquer.
• Não tenho nenhum livros. -> Não tenho nenhum livro. - nenhum and its noun are normally singular, even where English says “any” with a plural.
• Alguém sabem -> Alguém sabe - alguém, ninguém, tudo, nada are third-person singular and take a singular verb, however plural the meaning feels.

Tips

• Sort every indefinite on sight into one of two bins: it either agrees (algum, nenhum, todo, muito, pouco, outro…) or it is a frozen neuter pronoun (tudo, nada, algo, alguém, ninguém). That single split settles most agreement questions.
• The muito test: is it in front of a noun (agree: muitos) or in front of an adjective / verb (freeze: muito)? One word, two jobs.
• Learn the negatives as pairs with their affirmatives (alguém/ninguém, algo/nada, sempre/nunca), then apply the placement rule: a negative word after the verb keeps não; before the verb, drop it.
• Keep tudo (everything) apart from todo (all / every / whole), and todo o dia (the whole day) apart from todos os dias (every day).
• alguém, ninguém, tudo, nada are all singular - the verb stays singular, and their negatives feed straight into the negation you met in Lesson 15.