The trapassato prossimo is the easiest tense to use, in every language because it's always used the same way in any language. Our statement that there is no one to one relationship between the tenses of English tense system and the tenses of the Italian tense system must be corrected now a little bit now. There is a one to one relationship in the past perfect. The past perfect corresponds always to the trapassato prossimo. There is no problem from a practical point of view.
But there is a problem from a theoretical point of view. You can read everywhere that the past perfect is used to describe an action in the past that happened before another action in the past. This is nonsense. Let's have a look at this example.
He got on his car, lighted a cigarette, started the motor and drove away.
It is obvious that he got on his car before he drove away, getting on the car is an action that happened before driving away, but there is no need to use the past perfect. That leads us to the following question: When the past perfect must be used?
The past perfect must be used if the events are not told in the right chronological order.
a) They didn' t know any more where they were, they had gone round in circles for two hours.
b) They didn' t know any more where they were, they went round in circles for two hours.
In this case the semantic strength of the past perfect must be used. With semantic strength we mean the fact that the past perfect contains something that is normally expressed by adverbs like for example before. It is obvious that the two sentences above doesn't mean the same thing. In b) they didn't know where they were and so they went round in circles for two hours. In a) they didn't know any more where they were because before they had gone in circle for two hours and ended getting completely confused.
The past perfect must be used if an event is the logical consequence of another event.
a) Because his car had broken down, he was obliged to leave it there.
b) Because his car broke down, he was obliged to leave it there.
In this case the events are told in the right chronological order, but the simple past can't be used because it would change the meaning of the sentence. The simple past have a lot of function, between them the function to describe an action that happens regularly in the past, the meaning would be therefore: "Because his car (always) broke down, he was obliged to leave it there" or something like that.
The past perfect is to be used wenn an action had been finished in the past and the fact that it had been finished is the main statement of the sentence.
a) He had already written the letter but he didn't send it.
b) He already wrote the letter but he didn' t send it.
In this case a) is better than c) because the fact that he had written the letter and was not writing it is the main statement of the sentence.
a) He had not read the book, but he talked about it as if he had read it.
b) He didn' t read the book, but he talked about it as if he read it.
That's enough for the moment. Let' s discuss the formation and the use of the imperfetto, passato prossimo, trapassato prossimo and the passato remoto.