These are used to refer to one particular object or group of objects. In English, there is one definite article: the. Portuguese has four different definite articles used to match the number (singular or plural) and gender (masculine or feminine) of the noun.
The definite article is usually used before possessive adjectives and possessive pronouns.
a minha casa; os meus amigos; o livro é o meu
It is used before a title whether or not followed by a personal name.
O Senhor Ferreira tem razão; Como está A Sra.?
It is used before abstract nouns, as well as before some country and city names.
a generosidade; a Rússia; o Porto
It is used to make infinitives into nouns.
o escrever - writing
These are used when referring to a noun in a more general sense. In English they are: a, an, and some. Portuguese has four indefinite articles depending on the number and gender of the noun.
The noun after the indefinite article may be omitted, in which case the article is equivalent to English "one" or "ones".
quero um também - I want one too
quero uns frescos - I want fresh ones