Perfect forms (have + past participle) are used to express a connection between two moments in time, either an earlier and a later past event, a past event and the present, two moments in the future, or that something is/was/will be completed by a particular time.

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The structure for perfect forms is (have + verb in Past Participle), and thus, the structure for past perfect is (auxiliary verb 'have' in past = had + verb in past participle):

Had + verb in past participle

Examples:
She had visited him twice when his son appeared.
I had never seen that before, where did you get it?
They had traveled a million times before, but this was different.

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References:

AllGenius Drive

Cambridge

Exam English