The past perfect (or pluperfect) describes an action that was completed in the past, usually before something else happened.
The past perfect is formed by combining had with the past participle:
You had baked such a fantastic pie.
They had tasted success for the first time.
Simply add not:
We had not thought that possible.
Use the regular form:
Had he spotted the sniper?
I had noticed you were behaving oddly.
By the time the whistle went, we had scored five tries.
For more information, please see our introduction to all twelve verb tenses in English.