The definite articles accompany the noun to indicate that it is known, specific, or already identified in the context. In Portuguese, the basic forms are o, a, os, and as, and they agree with the gender and number of the noun. This class appears very frequently in noun phrases and also in fixed constructions linked to habits, titles, and proper names.

The four forms of the definite article vary only by gender and number. The choice depends on the noun they accompany, as in 'the book' and 'the tables'. This agreement is regular and predictable, and helps to clearly mark whether the referent is singular or plural and masculine or feminine.

IdeaIdea.	ExampleExample.
O artigo masculino singular é o.The masculine singular article is 'o'.	O livro está aberto.The book is open.
O artigo feminino singular é a.The feminine singular article is 'a'.	A mesa está limpa.The table is clean.
O artigo masculino plural é os.The masculine plural article is 'os'.	Os livros estão na estante.The books are on the shelf.
O artigo feminino plural é as.The feminine plural article is 'as'.	As mesas estão arrumadas.The tables are arranged.

The definite article agrees with the noun in gender and number, so its form changes when the noun changes. This agreement also appears when the noun is accompanied by an adjective or another modifier. In nominal sequences, the article continues to mark the known reference of the whole group.

IdeaIdea.	ExampleExample.
O artigo singular acompanha um substantivo singular.The singular article accompanies a singular noun.	A campainha tocou.The doorbell rang.
O artigo plural acompanha um substantivo plural.The plural article accompanies a plural noun.	Os alunos chegaram.The students arrived.
O artigo feminino acompanha um substantivo feminino.The feminine article accompanies a feminine noun.	A cidade cresceu.The city grew.
O artigo masculino acompanha um substantivo masculino.The masculine article accompanies a masculine noun.	O caminho é curto.The path is short.

When the definite article joins with certain prepositions, contracted forms of very frequent use arise. The most common combinations are de with o forming do, de with a forming da, and por with o forming pelo. These contractions belong to normal grammar of the language and appear both in writing and in speech.

IdeaIdea.	ExampleExample.
De com o forma do.Contraction of 'de' with 'o' forms 'do'.	O livro veio do armário.The book came from the cabinet.
De com a forma da.Contraction of 'de' with 'a' forms 'da'.	A chave caiu da bolsa.The key fell from the bag.
Por com o forma pelo.Contraction of 'por' with 'o' forms 'pelo'.	O menino passou pelo parque.The boy passed through the park.
Por com a forma pela.Contraction of 'por' with 'a' forms 'pela'.	A rua segue pela praça.The street runs through the square.

crasis occurs when the preposition a fuses with the feminine definite article a, resulting in à. It indicates this fusion in contexts such as 'vou à festa' and 'cheguei à escola'. Crasis does not occur before masculine words nor before personal pronouns, because there is no feminine article in those positions.

IdeaIdea.	ExampleExample.
A preposição a com o artigo a forma à.The preposition 'a' with the article 'a' forms 'à'.	Vou à festa.I'm going to the party.
A crase aparece antes de substantivos femininos determinados.Crasis appears before definite feminine nouns.	Cheguei à escola.I arrived at the school.
A crase não aparece antes de palavra masculina.Crasis does not occur before masculine words.	Fui a pé até o carro.I walked to the car.
A crase não aparece antes de pronome pessoal.Crasis does not occur before personal pronouns.	Entreguei a ela o recado.I gave her the message.

The use of the definite article before proper names varies according to region and register. In many varieties, the article is common with people's names, as in 'o João chegou', while in other contexts it can be omitted. This oscillation also appears in colloquial speech, which often reduces formal markers.

RegiãoRegion.	Palavra ou ExpressãoWord or Expression.	Definição RegionalRegional Definition.	ExemploExample.
BrasilBrazil.	O JoãoJoão.	O artigo pode aparecer antes de nomes de pessoa em uso regional ou coloquial.The article can appear before personal names in regional or colloquial use.	O João chegou, quando a conversa é informal.João arrived when the conversation is informal.
PortugalPortugal.	O JoãoJoão.	O artigo é muito frequente antes de nomes próprios em muitos contextos.The article is very frequent before proper names in many contexts.	O João entrou, quando a frase é natural na fala.João entered, when the sentence is natural in speech.
Registros formaisFormal records.	JoãoJoão.	O artigo pode ser omitido em estilos mais formais ou neutros.The article can be omitted in more formal or neutral styles.	João chegou, quando o texto pede neutralidade.João arrived when the text calls for neutrality.

With possessives, the article can appear in forms like my car or can be omitted, depending on the variety and level of formality. In generalizations and abstract concepts, the article tends to introduce the idea as a definite noun, as in happiness is important. In days of the week and habits, the contracted form à marks a regular repetition, as in às segundas eu corro.

IdeaIdea.	ExampleExample.
O artigo pode acompanhar possessivos em uso comum.The article can accompany possessives in common usage.	O meu carro está na rua.My car is on the street.
O artigo pode ser omitido com possessivos em registro mais neutro.The article can be omitted with possessives in a more neutral register.	Meu carro está na rua.My car is on the street.
O artigo introduz generalizações e conceitos abstratos.The article introduces generalizations and abstract concepts.	A felicidade é importante.Happiness is important.
A forma contraída pode marcar habitualidade com dias da semana.The contracted form can mark habituality with days of the week.	Às segundas eu corro.On Mondays I run.

The definite article appears with titles, positions, and place names when the reference is specific, as in the President or Brazil. In lists, vocatives, and certain titles, the article may be omitted to create a more direct or conventional utterance. The omission of the article is also an important feature in [Omission of the Article], while the differences between definite articles and [Indefinite Articles] help compare known reference and non-specific reference.

IdeaIdea.	ExampleExample.
O artigo pode aparecer antes de títulos e cargos.The article can appear before titles and positions.	O Presidente falou hoje.The President spoke today.
O artigo pode aparecer antes de nomes de lugares.The article can appear before place names.	O Brasil é grande.Brazil is big.
O artigo pode ser omitido em listas.The article can be omitted in lists.	Nome cidade país.Name city country.
O artigo pode ser omitido em vocativos.The article can be omitted in vocatives.	João, venha cá.João, come here.

The definite articles o, a, os, and as mark gender, number, and a determinate reference inside the noun phrase. They contract with prepositions, participate in crasis, can vary before proper names depending on the region, and appear in fixed uses with possessives, titles, places, and habits. When the language requires greater neutrality, omitting the article gains space, and the reading of this contrast becomes even clearer in comparison with [Indefinite Articles] and [Omission of the Article].