Past Perfect

Take a look at this example:

Paulina had written the letter before the class started.
Now she gave it to Mark.





The past perfect is used to refer to an event or situation which took place before another past event.

 

To help you to make it clearer:

more examples:

- I had lived in Spain for three years before I got married.

- She knew she hadn't studied it before the teacher called her to the blackboard.

And now compare the examples above with these similar examples in the present perfect. 

The only difference is that these examples are talking about the present:

- I have lived in Spain for three years. (until now)

- She knows that she hasn't studied at all, and now the teacher is calling her at the blackboard.

In these last examples we refer at the time of speaking, in the previous examples we refereed something before another event, therefor we used the past perfect.