What can you see from the following sentences?
1. By the time the tornado ended, more than 700people had been killed. 2. Coghlan travelled back to Canada after he had been buried in Texas.
动作在过去某一时刻之前已经完成, 且谓语动词与发出这一动作的主语存在被动关系。句中常用by, before, until, when等词引导时间状语。