Form and rules
How it works
Form the past perfect with had + past participle for every subject: I had left; they had finished.
Use it for the earlier of two past events when the order is important: When I arrived, they had left.
The later event is usually in the past simple.
Make negatives with hadn't + past participle and questions with Had + subject + past participle.
Do not use the past perfect automatically for every past action. If the order is already obvious, the past simple may be enough.