Past Tense: Agreement of the Past Participle

In the passato prossimo, the past participle of a verb conjugated with avere is invariable. The exception to this rule is when a third-person direct object pronoun (lo, la, li, le) precedes the verb phrase. In these cases, the past participle must agree with the pronoun in gender and number.

Take a look at the following examples which juxtapose cases not requiring agreement with cases which do require agreement.

No agreement:	Ho comprato la casa. I bought the house.
Agreement:	L’ho comprata. I bought it.
No agreement:	Abbiamo visto i bambini al parco. We saw the kids at the park.
Agreement:	Li abbiamo visti al parco. We saw them at the park.
No agreement:	Hai aperto le lettere? Did you open the letters?
Agreement:	Sì, le ho aperte. Yes, I opened them.

NB: The direct object pronouns lo and la are elided with conjugated forms of avere, but li and le never elide.