The past perfect (also called the pluperfect) refers to an action completed before another past action (usually in the same conversation). E.g.,
Aveva già mangiato quando sono arrivato.
He had already eaten when I arrived.
Erano già partiti quando abbiamo chiamato.
They had already left when we called.
However it can be used alone, without another verb in a past tense:
Non avevo mai visto Firenze.
I had never seen Florence (before this moment).
Nel 2011 non ero ancora iscritto all’università.
In 2011 I wasn’t enrolled in the university yet.
It is formed by the appropriate imperfect form of avere or essere, plus the past participle. Following are two examples: the past perfect of parlare and the past perfect of arrivare
Because of its comparative nature (with another past action within a given sentence), the trapassato prossimo is usually used alongside another tense (usually the passato prossimo, the passato remoto or the imperfetto). It can be used in the main clause or the subordinate clause of a given sentence:
Sabato scorso ho incontrato quel ragazzo che avevo conosciuto 2 anni fa al mare.
Last Saturday I met that guy whom I had met two years ago at the beach
L'estate scorsa, dopo che ero andata al mare sono andata in montagna.
Last summer, after having gone to the beach I went to the mountains.
The trapassato prossimo is often used with expressions like ancora, mai, già. These expressions are usually placed between the auxiliary (avere or essere) and the past participle.
Non avevo mai visto una città così bella.
I had never seen a city so beautiful.
Quando sono arrivato , il film non era ancora cominciato
When I arrived, the film had not yet started.
Erano già partiti.
They had already left.