The role of direct object pronouns with past participle

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In Italian, when the present perfect (passato prossimo) is made with the auxiliary verb essere, we have to make the past participle (participio passato) agree with the gender and the number (masc. sing., masc. plur., fem. sing. and fem. plur.). On the contrary, when the auxiliary verb is avere the past participle ends always in -o. But in any case (even if the auxiliary verb is avere), when we have a direct object pronuon the past participle must agree with the pronoun and ends with -o, -a, -i, -e.Example:

Hai comprato il giornale?   Sì, l’ho (lo ho) comprato stamattina
Hai comprato la rivista?    No, non l’ho (la ho) comprata!
Hai visto gli amici?             Sì, li ho visti ieri sera
Hai incontrato le tue amiche?    Le ho incontrate lo scorso sabato

But in case of direct object pronouns mi, ti, ci, vi, the past participle may or may not agree with the pronoun. Example, I can say: