ARTICLES:

Definite articles are used to determine the meaning of a noun:

Indefinite articles are used to generalize the meaning of a noun:

Pessoa is a feminine word.
Feminine words in Portuguese usually end in A:

Singular: a flor(the flower) / uma flor(a flower)

The definite article A and the indefinite article UMA are used before feminine nouns.
And if the noun is plural, we add an S to the article:

Plural:
as flores(the flowers) / algumas flores(some flowers)

Livro is a masculine word.
Masculine words in Portuguese usually end in O:

Singular: o livro / um livro
Plural: os livros / uns livros

*Attention to the masculine plural form of the indefinite article UM, you change it to UNS.

Although there are plenty of exceptions applied to this rule, in case you are not sure what article to use, you should follow the rule above.

Now check vocabulary for some exceptions:

Before names of countries and people’s names, we usually use definite articles:

Pay attention to the sentences bellow:

O Brasil é um país bonito.

Brazil is a beautiful country

O português é uma língua poética.

Portuguese is a poetical language

In Portuguese, we first mention what we are talking about (NOUN comes first), and then we qualify it (later the ADJECTIVE)!!!

A praia vazia, o dia quente, o escritório lotado, o ônibus quebrado: ninguém merece!!!

The empty beach, the hot day, the crowded office, the broken bus: nobody deserves it!!!

Before continents, states, oceans, mountains and rivers, we use the indefinite article:

It is very common to combine articles and prepositions:

De and do here combine de+a/o and it means the preposition from.

Na here combine em+a and it means in.

Sou de São Paulo, sou do Japão, sou da Inglaterra e do Paquistão! Eu sou um cidadão do mundo!!!