The indefinite articles introduce a new referent or one that is not identified precisely. In Portuguese, they also help indicate gender and number agreement with the noun they accompany. The chosen form depends on this agreement and the intention to quantify vaguely or to introduce an entity for the first time.

The basic forms are a, uma, uns and umas. They vary according to the gender and number of the noun, as in Gender of Nouns and Number of Nouns.

IdeiaIdea	ExemploExample
Forma masculina singular com substantivo masculino singularMasculine singular form with masculine singular noun	um carroa car
Forma feminina singular com substantivo feminino singularFeminine singular form with feminine singular noun	uma cadeiraa chair
Forma masculina plural com substantivo masculino pluralMasculine plural form with masculine plural noun	uns livrossome books
Forma feminina plural com substantivo feminino pluralFeminine plural form with feminine plural noun	umas ideiassome ideas

The indefinite article agrees in gender and number with the noun, and this agreement is mandatory in the standard form. When the noun changes number, the article also changes; when it changes gender, the final vowel accompanies the new form. This relationship is especially visible with countable nouns, as seen in Countable Nouns.

IdeiaIdea	ExemploExample
O artigo concorda com o substantivo no singularThe article agrees with the noun in the singular	uma notícia importantean important piece of news
O artigo concorda com o substantivo no pluralThe article agrees with the noun in the plural	uns amigos leaissome loyal friends
O artigo acompanha o gênero do substantivoThe article matches the gender of the noun	um problema difícila difficult problem

The indefinite article appears before the noun and can precede an adjective that accompanies it. The natural order is article, adjective and noun, as in a good idea. In informal registers, certain contexts may admit omissions or arrangements less expected, as also seen in Omission of the Article.

IdeiaIdea	ExemploExample
O artigo vem antes do substantivoThe article comes before the noun	um livro novoa new book
O artigo pode acompanhar um adjetivoThe article can accompany an adjective	uma boa soluçãoa good solution
A sequência natural mantém a ordem do grupo nominalThe natural sequence maintains the order of the noun phrase	umas pequenas mudançassome small changes

The indefinite article usually introduces a new entity in discourse. Therefore, it is frequent when something appears for the first time to the speaker or to the listener. After this introduction, other references may come to use more specific forms, often with a definite article.

IdeiaIdea	ExemploExample
Introduz um referente novoIntroduces a new referent	Vi um filme ontemI saw a movie yesterday
Apresenta algo ainda não identificadoIntroduces something not yet identified	Conheci uma professora novaI met a new teacher
Marca uma primeira mençãoMarks a first mention	Ela comprou um vestido azulShe bought a blue dress

The forms uns and umas can also indicate approximate or indefinite quantity. In this use, they do not mean exactly a fixed number, but suggest a small or imprecise set. This usage appears frequently in everyday expressions, close to indefinite pronouns and adjectives.

IdeiaIdea	ExemploExample
Indica quantidade aproximadaIndicates approximate quantity	Chegaram uns amigos tardeSome friends arrived late
Sugere número indefinidoSuggests an undefined number	Esperei umas horasI waited a few hours
Pode transmitir valor informal ou vagoCan convey informal or vague value	Tenho umas dúvidasI have some doubts

After 'to be', the indefinite article is often omitted with professions, jobs and identities. Thus, it is natural to say I'm a doctor rather than I'm a doctor when the idea is to classify the person by profession. When one wants to emphasize individuality, comparison or contrast, the article may reappear in certain contexts.

IdeiaIdea	ExemploExample
A profissão costuma aparecer sem artigo após serThe profession tends to appear without an article after 'to be'	Sou médicoI am a doctor
A identidade profissional fica mais direta sem artigoThe professional identity becomes more direct without an article	Ela é professoraShe is a teacher
O artigo pode aparecer para dar contraste ou valor particularThe article may appear to give contrast or particular emphasis	Ele é um ótimo médicoHe is a great doctor

In negative contexts, none and no tend to replace the indefinite article. Instead of pairing not with a or uma, the sentence tends to use the corresponding negative form. This pattern reinforces the complete absence of the referent and brings the sense closer to other negative indefinites.

IdeiaIdea	ExemploExample
Nenhum substitui um em negaçõesNone substitutes for the indefinite article in negations	Não tenho nenhum carroI don't have any car
Nenhuma substitui uma em negaçõesNone substitutes for uma in negations	Ela não tem nenhuma ideiaShe doesn't have any idea
A forma negativa enfatiza ausência completaThe negative form emphasizes total absence	Não vi nenhuma soluçãoI didn't see any solution

After prepositions such as in and for, the indefinite articles tend to contract. Thus, em can contract with the numeral um to num, and with uma to numa, and por can generate equivalent forms in common use, depending on the syntactic context. These contractions are part of the normal functioning of the noun phrase and are frequent in spoken and written language.

IdeiaIdea	ExemploExample
Em pode contrair com o artigo indefinidoIn can contract with the indefinite article	Estou num caféI'm in a cafe
Em pode contrair com a forma femininaIn can contract with the feminine form	Moro numa rua tranquilaI live on a quiet street
A contração aparece naturalmente no uso cotidianoThe contraction appears naturally in everyday use	Passei por umas lojasI passed by some shops

Before proper names, the indefinite article is not the normal behavior, but the definite article can appear in certain regional uses, especially in Brazil. This variation depends on the region, the context, and the degree of informality, as also occurs in other usage areas described in Definite Articles.

RegiãoRegion	Palavra ou ExpressãoWord or Expression	Definição RegionalRegional Definition	ExemploExample
BrasilBrazil	a MariaMaria	O artigo definido antes de nomes próprios é comum em várias regiões brasileiras.The definite article before proper names is common in several Brazilian regions.	A Maria chegou e sorriuMaria arrived and smiled
PortugalPortugal	o PedroPedro	O artigo com nomes próprios ocorre em certas variedades e contextos regionais.The article with proper names occurs in certain varieties and regional contexts.	O Pedro saiu cedo e voltou depoisPedro left early and returned later
Registo informalInformal register	artigo com nomearticle with name	A presença do artigo pode variar conforme a fala, o estilo e a região.The presence of the article can vary with speech, style and region.	A Ana ligou e deixou recadoAna called and left a message

The form um can function as an indefinite article or as a numeral. As an article, it introduces an unspecified referent; as a numeral, it expresses exactly the quantity 1. The syntactic context and the meaning of the sentence allow distinguishing these two values.

IdeiaIdea	ExemploExample
Um pode ser artigo indefinido quando introduz um referente novoUm can be an indefinite article when introducing a new referent	Vi um filme ontemI saw a movie yesterday
Um pode ser numeral quando significa exatamente 1 1️⃣Um can be a numeral when it means exactly 1	Comprei um livroI bought a book
O contexto define se há quantidade exata ou referência vagaThe context defines whether there is exact quantity or vague reference	Quero um caféI want a coffee

The indefinite articles present something new, agree with the noun and appear before it, and may accompany adjectives. The forms um, uma, uns and umas cover singular, plural, masculine and feminine, while none and no occupy this space in negations. There are also special uses with professions, contractions with prepositions, and regional variation in the presence of an article before proper names.