This is the 36th most frequent Romanian word.
ei
"They."
The word 'Ei' is used as a third-person plural subject pronoun meaning 'they', clearly identifying the subject of the sentence.
Ei sunt prietenii mei.
They are my friends.
The word 'ei' is used as a third-person singular feminine dative pronoun, indicating to whom the book belongs.
Carte aparține ei.
The book belongs to her.
The word 'ei' functions here as a third-person plural pronoun in the dative case, specifying whom I talked to.