Past perfect tense: The past perfect refers to a time earlier than before now. It is used to make it clear that one event happened before another in the past. It does not matter which event is mentioned first - the tense makes it clear which one happened first. when I arrived in the office.
When they arrived we had* already started cooking.
John had* gone out when I arrived in the office.
I had* saved my document before the computer crashed.
He was* very tired because he hadn't slept well.
Past tense: You can just use this to talk about the past general.