Forms: It had rained for weeks.
The basic meaning of the past perfect is 'earlier past': something had already happened at the time we are talking about.
During our conversation, I realised that we had met before.
The past perfect is common after past verbs of saying and thinking, to talk about things that had happened before the saying or thinking took place.
I told her that I had finished.
Past perfect or simple past : we use a past perfect, not a simple past, to say how long something had continued up to a past moment. A simple past perfect is used with 'non-progressive verbs' like be, have and know. With most other verbs, we use the past perfect progressvie for this meaning.
When they got married, the had known each other for 8 years.