The conjunction porque is used if the subordinate clause, the sentence introduced by the conjunctions, follows the main clause. If the main clause follows the subordinate clause, the conjunction is como.
como = since
Como não tinha dinheiro, não pude comprar o CD.
Since didn't have money, I couldn't buy the cd.
Como choveu, não fui a aula.
Since it rained, I didn't go to classes.
The same semantic value has the conjunction visto que. This conjunction can be used independently from the position of the main clause.
visto que = since / because
Visto que não tenho casa agora, ficarei aqui sem pagar renda até encontrar uma.
Since I have no place to stay right now, I keep staying here without paying until I have found something.
Ficarei aqui sem pagar renda até encontrar casa, visto que agora não tenho uma.
The conjunction uma vez que can trigger false associations among people who speak Spanish. One could think it means "at the moment when" or something like that, since that is what we get if we tranlate literally and what it actually means in Spanish. However this is wrong. With indicative it just means so that.
uma vez que = so that with indicativo
Uma vez que se trata de um problema mundial, requer também soluções mundiais.
Since it is a global problem, a global solution is needed.
Together with the conjuntivo uma vez que means what a literal translation suggests. In opposite to porque and como, that are always used with the indicative, uma vez que can be used as well with the conjuntivo.
uma vez que = when with conjuntivo
Uma vez que saibamos qual é a infecção, saberemos exactamente como a tratar.
When we know the kind of infection, we will know as well how to treat it.
Pues is in Spanish only an interjection: Pues, no sé que decirte. <=> Well, I don't know what to say. The actual use in Spanish is hard to define. Mostly it is used in Spanish to reinforce an assertion: "Pues, no!" <=> "Not at all!". In Portuguese it has become a conjunction.
pois = because
Vou morar na Argentina, pois lá trabalharei.
I will stay in Argentina, because I am working there.
It can be an interjection in Portuguese as well, but as an interjections the semantic value is as unclear as in Spanish. Sometimes we can translate with therefore.
interjektion pois ~ therefore
Existe, pois, uma situação perfeitamente inaceitável.
There is, therefore, a completely inacceptable situation.
