Definition :  The Past Perfect makes a relationship between an action that happened in the past and another point in time or action in the past.  The action conjugated in the Past Perfect is always the action that happened the earliest.

Example:     There are two actions that happened in the past.  We do not want to know when the actions actually occurred, but just that one happened before the other one.  Consider the situation where some thieves enter a house and commit a crime.  The robbers have time to leave before the police arrive.  We want to emphasise the fact that the police were too late.  In that case we must use the following sentence:

By the time the police arrived, the robbers had already left.

How to write the Past Perfect:

The Past Perfect is written using the Simple Past of the auxiliary To Have with the Past Participle of the verb you are conjugating.