All the perfect tenses should be taken in reference to the present. The past perfect is a tense used to describe an event that started and ended in the past. It is best used when the person has completed something in the past and there is no intention to carry that onto the future. It is often used in conjunction with another statement, which relates the progression of time like; “He had worked as a butler before coming to North America”.
The confusion lies when a person uses the past tense for the sake of simplicity and says, “He worked as a butler before he came to America”. Both actions are completed but it is clearer that he no longer intends to work as a butler and that is some thing that started and ended some time before coming over to America.
Like the present perfect it can be used for repeated events and accomplishments referring to the something done over a period of time in the past. He had worked been a member of parliament while he represented his constituency tells me about that accomplishment. He had been under attack from the opposition while he was a cabinet minister tells me that he faced objections while he held a government position. The past perfect can also be used in conjunction with the past continuous tense if the past event referred to occurred during another event that happened over a course of time in the past. So I can also say, “He had been a cabinet minister while he was representing his business clients”.
The past perfect tense should not be confused with its continuous form that is used to should a cause and effect. So if the person says he was upset because he had been waiting for a long time, this means that his emotional reaction was a result of his waiting. Both those actions occurred in the past, the waiting over a period of time before the emotional reaction. The use of the past perfect there should not be mixed up with using the past continuous form used together with the past tense, which is used to relate the fact that he reacted while or at time he was waiting: “He got upset while he was waiting”.
