Past perfect progressive: when to use it
先写下记得的内容，再查看解释。不要填写个人信息。
查看解释
The past perfect progressive highlights the duration or ongoing process of an activity that occurred before a later moment in the past. It shows that the earlier action was in progress up to or close to that second past event, often explaining a visible result or state at that time.
Elena had been painting the studio walls for two hours when the supervisor walked inside.