Articles are normally considered trivial and little attention is paid to them in grammar books. If we take a closer look on them they are not so trivial. Concerning learning Portuguese however they are not a big problem. Except some special contexts, there are no differences between Portuguese and English. If we use a definitive article in English, we use one in Portuguese, if we use an undefinitive article in English, we use an undefinitive article in Portuguese as well and if we use no article at all in English, we don't use any article at all in Portuguese.
The only differences are in the form. In Portuguese as in almost all languages of the world with only few exceptions like English, we have to distinguish between masculine and feminine, singular and plural.
The main difference between a definitive article and an indefinitive article can already deduced from the name. A definite article is used if we refer to something / someone knew and the indefinite article is used, if we refere to something unknown. If a man crosses the street, it is just any man and if the man crosses the street, it is a determined man. We can therefore produce contradictory sentences or sentences with a subtle meaning.
1) The man who won in the lotteries is my friend.
2) A man who won in the lotteries is my friend.
In case 1) we have an already long lasting friendship and his friend won in the lotteries. In case 2) we have a completely different situation. In this case any man can become his friend, provided that he won in the lotteries. Case 1) is a relative clause, case 2) is formally a relative clause as well, but in reality it is a conditional clause.
Im most languages the same word is used for one and a, but not in English.
one house = uma casa
a house = uma casa
There is actually no need for the indefinite article and some languages, like persian, don't have indefinitive articles. The indefinite article is used if we refere to something arbitrary. We get the same result by simply leaving it out.
1) I want bread. <=> 2) I want a bread.
Welll possible that 1) is a little bit rude and that the baker will look at us in an unfriendly way, if we enter in a bakery and just ask for bread, but the meaning is the same: We just want any bread. We want bread is something a starving crowd could cry in front of the palace of a king in the eve of a revolution.
However in other contexts we need something like an indefinite article.
1) Men sleep too much.
2) Some men sleep too much.
3) These men sleep to much.
In 1) the species man sleeps too much. In 2) some of this species sleeps to much and in 3) a determined group of man sleeps too much.
In some cases, if we refer to the species, there is no difference between an indefinite and an definitive article.
The human being is a curious animal. <=> Human beings are curious animals.
The definitive article is sometimes similar to the adjective demonstrativa (this, these, that, those).
1) I know the man.
2) I know this man.
In both cases we talk about a well defined man, although in case 2) the man is still more determined than in 1). But is we refer to a collective the definitive article is not possible.
1) The Romans wanted a bread and circuses.
2) The Romans wanted any bread and circuses.
3) The Romans wanted the bread and circuses.
4) The Romans wanted this bread and circuses.
5) The Romans wanted bread and circused.
1) doesn't make sense, because the indefinite article means that the kind of bread doesn't matter. This is actually true, but it doesn't make any sense to mention it in this context, because it is evident. If we use the indefinitive article we stress the arbitrarity, what is meaningless in this context as well. 2) has the same problem as 1), but is still worse. Any stresses the arbitrarity, but in the case that any kind of bread is as good as the other, it is absurd to stress that. 3) is the opposite of 2) and 1). The sentence means that we refer to a specific breand, what is not true and 4) is still worse. It would make sense if the romans had a preference for a certain bread, but that is not true. The only correct sentence is 5).
If there is an article, what is not the case in all languages, for instance there are no articles in latin, the language alle roman languages steems from, and in persian, the use of the articles resembles to the use in english. However sometimes there are differences. If one refers to a collective, to a totality of something, there is no article in English, but there is a definitive article in Portuguese.
Ich mag Kaffee.
I like coffee.
J'aime le café.
Me gusta el café.
Mi piace il caffè.
Gosto do* café.
We can perhaps say that the definitive article in roman languages doesn't point always to a determined thing or person. It can be used in roman languages in front of a proper name, ~The Mary went to the cinema <=> Mary went to the cinema, and in this context it doesn't make any sense, because Mary is already determined. Furthermore it is used in roman languages in front of collectivs, whereas in English that's not possible.
1) correct: Apple juice is made of apples.
2) wrong: The apple juice is made of apples.
However the definite article is to be used, if we refer only to a part of the collective. The sentence "Apple juice comes from Italy" would be wrong, only a part of apple juice comes from Italy and therefore it is "The apple juice comes from Italy" or "This apple juice comes from Italy". (We would say "The apple juice comes from Italy" for instance if there are several juices on a desk.)
In roman languages there is a definitive article in front of a collective.