Present perfect: when do we need to change the gender and number of the past participle in accordance with the subject?

Question

Example: «Siamo entrati senza la chiave».

I thought the passato prossimo form for entrare was entrato and that the conjugation should have affected only essere or avere. But apparently that is not the case. So what is the general rule here?

Answer 1

With intransitive verbs the past participle in the passato prossimo always agrees with the subject in its gender and number: siamo entrati, sono andata (if the speaker is a female) and so on.

For transitive verbs, the situation is trickier. In “normal”, subject-verb-object, sentences, the past participle is used in the singular masculine (Abbiamo mangiato la bistecca), independently on the subject and the object.

However, when the object is a pronoun and precedes the verb, the participle agrees with the object: l'ho mangiata (where l(a) refers to a bistecca), ci hanno battuti and so on.