what can you see from the following sentences?.

1.By the time the tornado ended, more than 700 people had been killed. 2.Coghlan traveled back to Canada after he had been buried in Texas. 3.My computer had been all right till last week.

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。