The past perfect tense indicates that an action occurred before another past action or time.
Indicates that the action started at some time in the past and continued until another time in the past (often used with for, since).
Used to express an assumption contrary to past facts (If... Had done...).
When the actions of both the main clause and the subordinate clause happened in the past, the past tense is used for the action that happened first, and the past tense for the action that happened later.
If you are simply describing a series of actions that occurred in the past (in chronological order), you can usually use the simple past tense.
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