The usage of the past perfect
Upon their entering the tomb, Carter’s lucky pet bird, which he had left in Cairo, was swallowed by a snake.
Carter left his lucky pet bird in Cairo.
Carter’s lucky pet bird was swallowed by a snake.
Past
Present
过去完成时表示过去某一时间或动作之前已经发生或 完成的动作。它表示动作发生的时间为“过去的过去”。