Tradução - Português brasileiro-Latim - Eternamente serei sua.

One little adjustment, sgrowl:
"In aeternum tuam ero".

We can use "suus, sua, suum" only when it is connected with third person and means his,her, its, their own.

One question... how can I distinguish in this sentence if "sua" is second person or third person?
Thanks in advance

Sorry, I don't understand the question. "Sua" is never for 2nd person, always for third person.

sorry, I was referring to portoguese term "sua"

Sorry for interferring...I'd like to make this clear.
In Brazilian Portuguese, the second person singular (tu) is seldom used in speech. It is substitued by "você" which takes the 3rd person form of the verbs, possessives, etc,
Yes, Aneta, although sometimes we can't identify exacly what person "sua/seu/suas/seus" refer to, in this text "sua" probably means "tua" (your-2nd singular)

Thank you, Lilly!
I had any doubts about what a pronoun should be here, I only wasn't sure about a gender. But Lizz told me it was feminine.

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One more thing, sgrowl. I didn't notice before you had used an accusative form of the pronoun. It has to be nominative and I'd change the order too:
"Tua in aeternum ero".
Do you agree?

Yes I do.
Even a little sentence hides some difficulties...

Yes, indeed.