Italians are all one big happy family, right? Maybe it's because there is so much cooperation in the Italian language. Plural nouns take plural articles, adjectives reflect the nouns they describe in both number and gender, and the past participles of verbs have a similar grammatical rule. When using the conversational past or other compound tenses, the past participle of the acting verb must agree in gender and number with the direct-object pronoun preceding the verb avere. Note the following examples:
Hanno visitato il nonno. (They have visited their grandfather.)
BUT: Lo hanno visitato. (They have visited him. Also: L'hanno visitato.)
Ho comprato i pantaloni. (I have bought the pants.)
BUT: Li ho comprati. (I have bought them.)
Abbiamo veduto Teresa. (We have seen Theresa.)
BUT: L'abbiamo veduta. (We have seen her.)
Ha ricevuto le lettere. (He has received the letters.)
BUT: Le ha ricevute. (He has received them.)
From:https://www.dbfei.com/Article/yidaliyu/201311/7715.html