[Originally posted on 08/01/14 on the Duolingo French for English speakers forum by TheMotley]
The passé composé is the most common French past tense, therefore you’ll need it every time you want to write in the past.
Luckily, you only have two auxiliary verbs to pick up from (avoir and être) but how to choose?
Firstly, most of French verbs use the auxiliary verb avoir, whilst the auxiliary être is used in some particular cases.
e.g.
J’ai fait mes devoirs (I did me homework)
Elles ont gagné le match (They won the game)
But when to use the auxiliary être?
In most of the cases, the auxiliary verb être is used with pronominal verbs (se taire, se laver, se rencontrer, s’aimer…etc) and action verbs (sortir, entrer, aller, revenir, tomber…etc).
e.g.
Hier, Je ne suis pas allé à l’école (Yesterday, I didn’t go to school)
Il
s‘est lavé les mains avant de manger (He washed his hands before eating)
! With the auxiliary *être*, the conjugated verb accords with the gender and type of the phrase subject.
e.g.
Elles sont allées à l’école (They went to school)
Vous vous êtes levés . (You woke up)
! All transitive verbs uses avoir as auxiliary verb. Some verbs that generally use être can be transitive in some cases and the most common example for that is the verbe tomber.
e.g.
transitive: Il
atombé sa veste (He dropped his jacket)intransitive: La nuit
esttombée (The night fell)
I hope this was helpful.
Selected comments on original post
(Devenir Revenir
Monter Rester Sortir
Venir Aller Naître Descendre Entrer Rentrer Tomber Retourner Arriver Mourir Partir)
Actually, Passer and Descendre depend on the context. And also tomber, but I’ve explained it in the thread.
e.g.1
Je suis passé chez toi hier soir, mais tu n’étais pas là.
J’ai passé mon temps sur l’ordinateur.
e.g.2
Je suis descendu à la cave pour chercher une bonne bouteille.
J’ai descendu les escaliers à toute vitesse, j’ai bien failli tomber.
Bon courage!
Good, but I’d just like to know: is there any difference in the choice of auxiliaries in french and in italian? It would simplify my mental patterns quite a lot 
Sorry, I don’t speak Italian I can’t help you with that :s you seem to be a Southern Italian though, aren’t you? :p
A transitive verb in French is a verb that requires one or more direct objects. This contrasts with intransitive verbs, which do not require objects.
Duolinguists 