Choose the past perfect or the past simple. 6. When we (arrive) at the station, the train had already left.

{ "answer": "6. When we **arrived** at the station, the train had already left.

**Explanation:**
* 'Had already left' is in the past perfect, indicating an action completed before another action in the past. Therefore, the action of arriving at the station happened after the train had left, so it's in the past simple." }