Italian Perfect Tenses

What are Perfect Tenses?

A perfect tense is a compound tense which combines an auxiliary verb with a past participle. For every simple tense, there is a corresponding perfect tense. Here are a few examples from English:

Present: I eat
Past: I ate
Present Perfect: I have eaten
Past Perfect: I had eaten

In English, the auxiliary verb is always "to have". In these examples, the participle is "eaten."

In Italian, the auxiliary is either avere or essere. 

Present: mangio
Past: mangiavo
Present Perfect: ho mangiato
Past Perfect: avevo mangiato

In the last two, mangiato is the participle.

Present: vengo
Past: venivo
Present perfect: sono venuto
Past perfect: era venuto
(I come, I came, I have come, I had come)

In the last two, venuto is the participle.

When to use essere in Italian?

There are three fairly solid rules that cover most verbs:

1. All transitive verbs use avere. That is, if the verb has a direct object, it takes avere. 
2. All reflexive verbs use essere. 
3. Simple verbs of motion take essere. Go, come, etc.

Beyond this, I'll paraphrase Maiden and Robustelli (section 14.20). For intransitive verbs, essere tends to be used for verbs that emphasize a final state rather than the process that led to the state and for those whose subjects had little to do with controlling the action. So cambiare (to change) takes essere as does annegare (to drown).

Intransitive verbs that take avere tend to be ones whose subjects play a major role in the action or state of the verb. So agire (to act) and nuotare (to swim) both take avere.

When does the Participle Change for Number or Gender?

In the example above, "I have come" was sono venuto, but that only works if I'm a man. A woman would say sono venuta. For a group of people, we would say sono venuti "they have come" unless the group was all women, in which case we'd say sono venute. On the other hand in the case of eating (as an intransitive verb), the participle never changes. Ho mangiato, ha mangiato, hanno mangiato, etc.

So what's the rule?

Whenever the auxiliary is essere the participle always agrees with the subject. As I just illustrated, this is true even when the subject is "I."

When the auxiliary is avere the participle doesn't ever change for an intransitive verb. When the verb is transitive, the following applies:

1. If the direct object is one of the five clitics (pronouns) lo, la, li, le, ne, then the participle must agree with the object (not the subject).
2. For other direct-object clitics (mi, ti, ci, vi) agreement is optional.
3. If the direct object is a noun, then the participle never changes, although you may see it in older literature.

There are some good examples on about.com, although the explanation isn't (as of September 2014) up to their usual standards.