"The wallet is ours."
Translation:A carteira é nossa.
Well, "ours" can be singular or plural in English. "Ours" is the genetive(possessive) case of "we", used to establish relationship(possession). In this sentence, it is intended that the word is singular which is distinguishable in Portuguese "nossa" as opposed to "nossas", but in English it remains ambiguous in terms of number(sing./plural). Hopefully, the context will help in such situations.