Past perfect (had + participle)

Form: had + past participle for all persons (I had gone, she had seen). Continuous: had been + -ing.

Use: “the past before the past” — when you are already telling a story in the past and go further back.

When we arrived, the film had already started. (first the film started, then we arrived)
When we arrived, the film started. (we arrived, then it started)

She was tired because she had been working all day. — I had never seen snow before I moved to Switzerland.

It is also needed in conditional type 3 (If I had known…), after wish about the past (I wish I had studied), and in reported speech (He said he had finished). If the order is clear from before / after, the past simple is enough: I left after he arrived.