You use the Past Perfect generally to talk about an action that occurred before one expressed in the Past Simple.
- He had already gone by the time I arrived. (First action comes first chronologically)
-We went to see a movie I had already seen. (Second action comes first chronologically)
January 23, 2016
When you explain the reason, or background, for an action in simple past tense.
This is pre-intermediate grammar - we certainly can't ignore it! :)