if you wanted to say your dogs instead of your dog, it would be "teus cachorros?"
Brazilian natives... could it be "your puppy"? in spanish "cachorro" is puppy and "perro" is dog. I tried puppy and got it wrong
yes, it differs from Spanish. Portuguese: cachorro/cão = dog, cachorrinho/filhote de cachorro = puppy.
Are they both used in Brazil? Is one used in certain instances that the other isn't?
teu = your, seu = your, his, her, its, their. Seu is more common than teu.
How do you choose when to use "teu" or "seu"? and whether "seu" is referring to "his" or "her"? Context?
In Portugal (and some parts of Brazil, I guess), you would use «teu» whenever you would address the person as «tu» (friends close to your age, close family members, etc.) and «seu» whenever you would address the person as «você» (higher rank/older than you) or when you are talking about him/her. Yes, context resolves everything. «O Tiago deixou o seu casaco em casa.» = "Tiago left his coat at home."
if you address the person with você you should use "seu" (since grammatically it's a third person construct, like ele/ela). If you address them with "tu" then you should use "teu".
As far as I understand "tu" is fairly uncommon in most places in brazil so they probably don't use "teu" very much.
In Brazilian Portugese, would it be correct to say "É um teu cachorro" (it is your dog)? Or is teu more used in Portugal? I think I recall hearing that somewhere. If that is correct, then what would Brazilians say? "É um seu cachorro"? Or "a seu"? Or am I completely on the wrong track here?
first of all, it would be "É teu cachorro". But the truth is that even brazilians get confused with teu/seu. I think in the south part of Brazil people say more "teu" than the rest of the country but it means nothing. The advantage of knowing which one to use is to avoid ambiguity. "teu" only means your, meanwhile "seu" could be used to your, his, her, its.
When do you have to put in the "o"? I notice when it is plural it would say "os teus cachorros". So, could this also be "o teu cachorro"?
Yes. Most of the time, there's no difference. As an advice, we usually put the article. But sometimes they're different:
Ele é meu fã = He is my fan
Ele é o meu fã = He is my fan (when he is already known/mentioned in speech, or he is my only fan!) ='(
Hi Paulenrique, I find all of your comments helpful, however (and I may be doing it wrong) I can not click on the links you provide. I've seen your comments on other questions and your responses are great, but every time I click on your link it takes me back to my duo lingo homepage. Anyone else having issues or just me?