"The soup is for the captain."
Translation:A sopa é para o capitão.
when would I use por instead of para? I used para and it was correct. Would por or pelo also work here?
No, neither "por" nor "pelo" would work there. :-/
"Para" is really "to" (or another preposition whose meaning thread more or less that particular nuance -- like "for", when you make something to or for someone). Examples: 1) A sopa é para você (the soup is for/to you) 2) Quero esse livro para mim (I want this book for myself)
"Por" means "via", "through" (by, per etc.), or any preposition that conveys this particular sense (like "for" when you say "I long for a rest" -- eu anseio por um descanso). Examples: 1) É melhor por aqui (it's better this way) 2) Ele foi condenado por ter roubado (he was sentenced for stealing)
Or in expressions, such as "that's why" ([é] por isso): 1) É por isso que não confio em você (that's why I don't trust you)
"Pelo" (or pelos, pela, pelas) stands for the preposition "por" + article "o" (or articles "os", "a", "as" respectively). Example: 1) Ele foi pelo caminho do meio ("he took the middle path", or more literally, "he went through the middle path"). 2) A julgar pelo tamanho, creio que seja perigoso (judging by the size of it, I believe it's dangerous)
I see your (sur?)name is chaves? If you're a Spanish speaker, it's pretty much the same in your language:
> La sopa es PARA el capitán.
As opposed to: "me voy POR EL camino del medio".
"Para" and "por" work nearly the same way in Portuguese and Spanish.