In English "your" (possessive adjective) is used only before nouns like in "This is your cake". On the other hand, "yours" (possessive pronoun) is used only alone, without a noun like in "The cake is yours". It's the same for:
- my & mine
- your & yours
- her & hers
- our & ours
- their & theirs
Only "his" and "its" are used without any change, because they end in -s anyway.
"O bolo é seu" and "O bolo é o seu" have slightly different meanings.
"O bolo é seu" is like "Do you see this cake? It's yours!"
"O bolo é o seu" is more like "The cake is that cake of yours we were talking about". ("O seu" implies we are talking about something that has already been referenced previously.)