Like every student of Italian, at some point or another you're going to be faced with having to remember which verbs take Essere and which take Avere as their helping verb in compound verb tenses like the Passato Prossimo and the Trapassato Prossimo. I've put together a little explanation here to help you guys out.
Here's how to know whether to use Essere or Avere as the helping verb: Avere is used for Transitive verbs, verbs where actions pass directly from the subject (actor) to the object (the receiver of the action). For example, mangiare (to eat) is a transitive verb. When I say "I had eaten an apple" - "Avevo mangiato una mela," the action of "to eat" was directly passed from me as the actor to the apple as the object. (The majority of verbs take Avere as their helping verb).
Essere is used for Intransitive verbs, verbs where actions cannot pass from the actor to the object. As we're dealing with Essere, we need to be mindful of gender and number. Intransitive verbs are commonly referred to as "walking verbs" for they usually have something to do with motion. Andare (to go) is an intransitive verb and it deals with motion, just like arrivare (to arrive). I always tell my students this: I can show you the action "to eat" but I can't show you the action "to arrive." Maybe this little explanation will help some of you to understand the difference between Transitive and Intransitive verbs better. Either way, you can find a list of some common Intransitive verbs that all take Essere as their helping verb here:
Andare - to go
Arrivare - to arrive
Cadere - to fall
Diventare - to become
Durare - to last
Entrare - to enter
Esistere - to exist
Morire - to die
Nascere - to be born
Occorrere - to occur
Partire - to leave/depart
Restare - to rest
Rimanere - to remain
Ritornare - to return
Scappare - to escape
Stare - to stay/to be
Svanire - to vanish
Tornare - to return/come back
Uscire - to exit
Venire - to go
*Also keep in mind that Reflexive Verbs tend to use Essere as their helping verb as well as verbs that behave similarly, such as piacere (to like), mandare (to send), servire (to need).
I hope this helps! Keep up the great work and always remember to SPREAD THE LOVE!
Tom