passato prossimo with avere as the auxiliary

Making the past participle agree with direct object pronouns in the past: If you use a direct object pronoun (lo, la, li, le) with a verb that takes avere as its auxiliary verb, you must agree the past participle with the direct object pronoun: Ho mangiato la pizza. L'ho mangiata. Notice how mangiato becomes mangiata when you change the pizza into a feminine pronoun. Agreement with mi, ci, ti, and vi is optional.

(examples)

Ho mangiato il pomodoro. > Lo ho mangiato. (often elided to "L'ho mangiato".)

Ho mangiato la pizza. > La ho mangiata. (often elided to "L'ho mangiata".)

Ho mangiato le fragole. > Le ho mangiate.

Ho mangiato i piselli. > Li ho mangiati.

In the passato prossimo, the past participle never has to change to agree with the indirect object pronoun.