PAST PERFECT = HAD + PARTICIPLE
The SIMPLE PAST indicates a simple past action: I arrived at the bus stop.
The PAST PERFECT indicates a previous action before the simple past:
The bus had left when I arrived at the bus stop.
IN A NUTSHELL: The past perfect is a past in the past.
Other examples:
-When Wendy arrived (9:15), the class had started (8:30).
-When I arrived at the bus stop (7:32), the bus had left (7:30).
-When I turned on the TV, the film had ended.
