This is the 911th most frequent Serbian word.
њеним
"Њеним" means "her" or "hers" (instrumental or dative case) in English.
In this sentence, 'њеним' is the instrumental plural form of the word 'њен' (her) and is used to specify through what or where the action of arranging occurred, referring to the girl's rooms.
Сложили смо намештај у њеним собама.
We arranged the furniture in her rooms.
Here, 'њеним' is the locative plural form of 'њен' (her), appearing after the preposition 'о' (about), and describes to whom or what the subject 'успеси' (successes) belong in the context of the story being narrated.
Послушај причу о њеним успесима.
Listen to the story about her successes.
The word 'њеним' here is the dative plural form of the word 'њен' (her) and is used to indicate the indirect object of the verb, referring to the parents of the girl being discussed.