The idea
Use the past perfect to look back from a moment in the past. It makes the earlier of two past events clear.
Form
had + past participle
Use the same form with every subject.
Positive: They had watched three episodes.
Negative: They hadn’t watched the final episode.
Question: Had they watched the trailer?
Two events in the past
The past perfect normally describes the earlier event. The later event is often in the past simple.
The episode had already started when we turned on the television.
We chose a different series because we had seen the first one before.
By the time our friends arrived, we had watched three episodes.
Words such as already, just, before, after and by the time often help make the order of events clearer.