This is the 189th most frequent Croatian word.
svojoj
"Svojoj" means "to one's own" or "to her/his own" in the dative case.
Here, 'svojoj' is used as a dative form of the reflexive possessive pronoun, referring to something belonging to the subject Petra.
Petra piše pismo svojoj prijateljici.
Petra is writing a letter to her friend.
In this sentence, 'svojoj' is again in the dative form, as it describes the relation to the verb 'pomaže' and refers back to the subject Ivan.
Ivan pomaže svojoj sestri s domaćom zadaćom.
Ivan is helping his sister with homework.
In this example, 'svojoj' modifies 'majci' and points out that the mother is related to the subject Ana, in the dative case indicating the indirect object.