How do I form the trapassato prossimo (pluperfect) tense in Italian? When do I use it?

The pluperfect in Italian is a composite (or if you want to sound fancy, a periphrastic) tense, like the passato prossimo (e.g. ho finito, 'I have finished'), so we make it by combining an auxiliary verb with a past participle. In this case, it's:the imperfect form of the auxiliary avere or essere + the past participle. So 'I had eaten an apple' would be avevo mangiato una mela. 'You had eaten an apple' would be avevi mangiato una mela. With regards to when the trapassato prossimo is used, you can see from my examples that it's pretty similar to the pluperfect tense in English; we use it to refer to a past action that precedes another past action. In other words, it demonstrates a sequence of events. So, it's not just that 'I had eaten an apple' FULL STOP, but that 'I had eaten an apple when I left': avevo mangiato una mela quando sono partita. Just remember here as well that if the auxiliary you are using is essere (as in sono partita) you must make the past participle agree with the gender of the subject(s) carrying out the verb. There are some other things to remember regarding use of the subjunctive (gets a bit complicated!) and subject changes, but first we'll keep it simple and practice the TP with some different examples.