Past perfect is used to show one thing in the past happened before another thing in the past. This other thing in the past is usually expressed by past simple. eg
The train had left when I arrived at the station.
Had left is past perfect and this event happened before arrived which is past simple.
So you would use past perfect when telling a story about things in the past. eg
I felt really tired when I took the train to work yesterday because Sophie and I had been to a party the night before. We hadn't gone to bed until 3:00am. I had been on the train 5 minutes when I realised that I had left my wallet at home. I phoned Sophie but she had already left for work.
A writer should use the past perfect tense to indicate an action that occurred before another action in the past. It is useful for showing the sequence of events or emphasizing the completion of an action before another started.
The past tense of "exist" is "existed." The past perfect tense is "had existed."
The past perfect tense of hope is had hoped.
The past tense of "bleed" is "bled." The past perfect tense of "bleed" is "had bled."
The past tense of sit is sat. The past perfect tense of sit is had sat.
present tense past tense future tense present perfect tense past perfect tense future perfect tense present progressive tense past progressive tense future progressive tense present perfect progressive tense past perfect progressive tense future perfect progressive tense
The past perfect tense of 'get' is 'had gotten'
Perfected is the past tense of perfect.
"Had been" is the past perfect tense of be.
The past perfect tense of "do" is "had done." For example, "She had done her homework before dinner."
The past perfect tense of borrow is "had borrowed."
The past perfect tense of "think" is "had thought."
The past perfect tense of say is had said.