If your verb does not fit into one of those categories listed above (change of location; staying in location; change of state), then you will most likely use avere as your auxiliary verb to form the present perfect. Accordingly, the majority of Italian verbs take avere as the auxiliary verb.
When you form the present perfect (passato prossimo) with avere (usually for transitive verbs, i.e., verbs that take a direct object), you do not have to match your past participle with the gender and number of the subject. E.g.,
I ate the sandwich. Ho mangiato il panino.
We ate the pizza. Abbiamo mangiato la pizza.
She ate the salad. (Lei) ha mangiato l’insalata.
Following is an example of a verb, comprare (to buy), fully conjugated in the passato prossimo:
A definition of transitive:
Just above we said that you will use avere to form the present perfect with transitive verbs. Following is a more detailed definition of transitive verbs:
A transitive verb is a verb that allows a direct object. For example:
Io ho mangiato (che cosa?) la pizza.
I ate (what?) the pizza. (“The pizza” is a direct object.)
Tu hai incontrato (chi?) Gustavo.
You met (whom?) Gustavo. (Gustavo is a direct object.)
In the two examples above, mangiare and incontrare are both transitive verbs. As mentioned at the beginning of this section, the majority of Italian verbs form the present perfect with avere.
Irregular past participles in the present perfect
There are many irregular past participles, especially for verbs of the second conjugation ( –ere verbs). Following is a list of the most commonly used verbs which have an irregular past participle.
-are
fare
to do, to make
fatto
-ere
*Attenzione! The past participle of essere is stato, which is also the past participle of stare.
-ire