Mas and porquê don't mean the same thing. Only "but" can be translated as "mas", whereas "porque" is actually translated as "because". "Mas" is used to show contrast between two sentences and "porque" is used to explain something, as a result. Ex: Estou cansado, mas não trabalhei hoje. I'm tired, but I didn't work today. Estou cansado porque trabalhei hoje. I'm tired because I worked today. Tirou uma nota boa, mas não estudou. He got a good grade, but he didn't study. Tirou uma nota porque estudou. He got a good grade because he studied.

"Mas" means "but". Also can mean "however", "nevertheless". "Porque" means "because"

Hello Halalau, Using 'mas' as 'porque' expresses a sort of excuse for the sentence which precedes it, an assumption; usually followed by the verbs ‘dever’ and ‘poder’ . For instance: He has not talked to me; he might not have seen me. Ele não falou comigo; mas não deve ter me visto. If I had used ‘porque’ it would sound as if I was sure he had not seen me. Although, I am not sure all the native speakers would tell the difference. Normally I’d say “Ele não me viu, deve estar cansado’ in this particular case. To me, this is the only case they have similar meanings. I have double checked that with the Houaiss dictionary of Portuguese language (2009), in which this is the least common acceptation for ‘mas’. For other cases ‘porque’ is related to because, for or as.

the word "mas" is a adversative conjunction, used as a oposition between two sentences exemple: Estou cansado, mas preciso estudar. Ele falou alto, mas ninguém o ouviu. the word "porque" is a conjunction to explains "why" of the sentence ex: Eu estudei a noite toda porque tive prova. Eu acordo cedo porque vou para o trabalho ps: they don't mean "before" in either cases