past perfect (I had done)
Study this example:
Sarah went to a party last week. Paul went to the party too, but they didn't see each other. Paul left the party at 10.30 and Sarah arrived at 11 o'clock.
When Sarah arrived at the party, Paul wasn't there.
He had gone home. Had gone is the past perfect.
The past perfect simple is had+ past participle (gone/seen/finished etc).
Sometimes we talk about something that happened in the past:
Sarah arrived at the party
This is the starting point of the story. Then, if we want to talk about things that happened before this time, we use the past perfect (had ... ):
When Sarah arrived at the party, Paul had already gone home.
Compare the present perfect (have seen etc.) and the past perfect (had seen etc.):
Compare the past simple (Left, was, etc.) and the past perfect (had left, had been, etc.):