过去成时表示过去某时之前已完成的动作或状况。在时间上, 它属于“过去的过去”。在句中常有明显的参照动作或有表示“到过去某时为止”的时间状语。

　　By the end of the war, the small workshop had become a large factory.

　　They found that a stream had formed in the field.