Questions & Answers about Ho comprato un libro scontato.
In Italian, most transitive verbs—verbs that take a direct object—use avere to form the passato prossimo. Since comprare (to buy) directly takes un libro without a preposition, you combine the present of avere with the past participle:
• avere (ho) + comprato = ho comprato
By contrast, essere is reserved for unaccusative verbs (e.g. arrivare, nascere) and reflexive constructions.
Passato prossimo = present tense of avere + past participle of the main verb.