There are two past tenses used to describe things that happen before another point in time in the past. Use the past perfect to talk about an event which had happened at some point in time before something took place.
She had already eaten by the time they arrived.
Past Perfect Structure
Subject + had + past participle
I, You, He, She, We, They had finished before I arrived.
Subject + had + not (hadn't) + past participle
I, You, He, She, We, They hadn't eaten before he finished the job.
Question word + had + subject + past participle
What - had he, she, you, we, they thought before I asked the question?
Listed below are uses with examples, and the structure of the past perfect tense.
Something that had happened before something else took place
I had eaten lunch before they arrived.
Susan had already finished the project when he asked for help.
Something that had happened over a period of time in the past before another point in the of past
I had lived abroad for twenty years when I received the transfer.
Jane had studied in England before she did her master's at Harvard.
Something that had happened as a precondition to something else
I had prepared for the exams and was ready to do well.
Tom had lost twenty pounds and could begin anew.
In the third conditional to express imagined conditions
If I had known that, I would have acted differently.
She would have come to the party if she had been invited.
With wish to express a desire about the past
I wish you had told me.
She wishes she had known about his problems.
Common time expressions used with the past perfect:
already, just, before, when