Past Perfect is formed by means of the auxiliary verb had and Past Participle (3rd form of irregular verbs or ending -ed for regular verbs).
For example:
I had done it by the time he came home.
They had done it by the time he came home.
He had finished by the time we came.
I had not done it by the time he came home. (hadn’t done)
They had not done it by the time he came home. (hadn’t done)
He had not finished by the time we came. (hadn’t done)
Had they done it by the time he came home?
Had they done it by the time he came home?
Had he finished it by the time we came?
Cases of using:
1. Past action which occurred before another action or before a stated past time
He had left by the time I got there, (or by 8.15)
2. Complete past action which had visible results in the past
She was sad because she had failed the test.
3. The Past Perfect is the past equivalent of the Present Perfect
(He can’t find his watch He has lost it.)
He couldn’t find his watch. He had lost it.
Time expressions used with:
or, since, already, after, just, never, yet, before, by, by the time etc.