Past Perfect Simple: Action Before Another Past Action

Past Perfect talks about an action before another past action. Form: had + V3 — for every person.

• When I arrived, the train had left. — the train left first
• She had already eaten, so she wasn't hungry.
• I hadn't seen the film before that night.

If order is obvious with after / before, Past Perfect is not always required, but it is clear when two past moments contrast.

Do not use Past Perfect as a “smarter Past Simple” for a single event: I went home at 10.