This is the 172nd most frequent Romanian word.
nostru
"Our" (possessive adjective).
In this sentence, 'nostru' serves as a possessive pronoun to indicate that the dog belongs to a group including the speaker.
Acesta este câinele nostru.
This is our dog.
Here, 'nostru' is again used as a possessive pronoun to denote the teacher associated with the speaker's group.
Profesorul nostru ne-a dat un test.
Our teacher gave us a test.
'Noastră' is the feminine singular form of the normative possessive pronoun, agreeing with the noun 'casa' (house) in gender, number, and case.