This is the 583rd most frequent Serbian word.
другу
The Serbian word "другу" is the dative or accusative case of "друг," meaning "friend" (male).
Here, 'другу' is in the dative case, used to indicate the indirect object of the sentence, showing with whom the subject (I) went to the park.
Ишао сам у парк са свом најбољем другу.
I went to the park with my best friend.
'Другу' appears in the dative case again, showing to whom the book was returned, serving as the indirect object of this action.
Књигу сам вратио свом другу.
I returned the book to my friend.
In this sentence, 'другу' also in the dative case specifies to whom the explanation was directed, functioning as the recipient of communication.