Do I use essere or avere in the passato prossimo?
Most verbs take avere. Essere is taken by verbs of motion and change of state, by reflexives, and by a handful of common intransitives like stare, piacere and essere itself. When a verb takes essere, its past participle agrees with the subject in gender and number; with avere it normally does not change at all.
Avere is the default
Any verb with a direct object takes avere, and so do most intransitive verbs of activity. Start from avere and treat essere as the marked case with a learnable list.
Ho mangiato una pizza.
I ate a pizza.
Abbiamo parlato a lungo.
We talked for a long time.
Hai dormito bene?
Did you sleep well?
Essere for motion, change of state and reflexives
Andare, venire, arrivare, partire, entrare, uscire, nascere, morire, restare, diventare — going, coming and becoming. All reflexives take essere too, whatever the underlying verb would take on its own.
Sono andata a Roma.
I went to Rome.
Female speaker — andata.
Siamo arrivati tardi.
We arrived late.
Mi sono lavato.
I washed.
Reflexive, so essere.
Some verbs take either, with different meanings
A group of verbs switch auxiliary depending on whether they are used transitively. With an object they take avere; without one they take essere, and the meaning shifts accordingly.
Ho cambiato idea.
I changed my mind.
Transitive — avere.
Il tempo è cambiato.
The weather changed.
Intransitive — essere.
Ho finito il libro. / Il film è finito.
I finished the book. / The film ended.
Common mistakes
Ho andato al cinema.
Sono andato al cinema.
Andare is a verb of motion and always takes essere.
Sono mangiato una pizza.
Ho mangiato una pizza.
Mangiare takes a direct object, so avere — and the participle does not agree.
Maria è andato a casa.
Maria è andata a casa.
With essere the participle agrees with the subject, which is feminine here.
Frequently asked questions
Is there a rule or just a list?
Mostly a pattern with a list around it. Motion and change of state take essere, transitive verbs take avere, and reflexives always take essere. The awkward cases are the intransitives that do not obviously involve motion — piacere, stare, costare — and those are memorised.
Does French work the same way?
Closely, and if you know French the intuition transfers well. The lists are not identical, though, so do not assume a verb behaves the same in both.
What happens with modal verbs?
Formally the modal takes the auxiliary of the verb it governs — sono dovuto andare. In practice many speakers use avere throughout, and ho dovuto andare is very widely heard and accepted.