Choose the past perfect or the past simple. 12. When he (arrive) at the party, Julie had just left.

{ "answer": "12. When he **arrived** at the party, Julie had just left.

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