Semantically mas and porem are equivalent, but porem is more flexible concerning the position in a sentence. Mas can only be used at the beginning of the subordinate clause, porem can stay as well after the subject of the subordinate clause. There can be a subtle difference in the meaning.
| mas is always at the beginning |
| Ninguém respondeu à pergunta, | mas |
os alunos sabiam a resposta.  |
| Nobody answered the question, |
but |
the students knew the answer. |
| porém can stay as well after the subject of the main clause |
| Ninguém respondeu à pergunta; os alunos, | porém, |
sabiam a resposta. |
| Nobody answered the question, the students |
however |
knew the answer. |
| Tentei chegar mais cedo, porém não consegui. |
| I tried to come earlier, however I didn't made it in time. |
| Podem ficar, porém este estabelecimento fecha às sete. |
| You can stay, however the building closes at seven o'clock. |
| Existem, porém alguns sinais encorajantes. |
| There are however some encouraging signals. |
| O comércio global não deve, porém ser desvirtuado. |
| The global trade shoudn't be attracted. |
