用于某些特殊句型。如：
This was the first time he had ever been late. 这是他第一次迟到。(类似 it was the first time that…后的从句谓语要用过去完成时)
It was three years since we had left the city. (那时)我们离开那座城市已有三年。(类似 it was…since…后的从句谓语要用过去完成时)
We had no sooner set out than a thunderstorm broke. 我们刚出发就遇到了大雷雨。(no sooner…than 的主句谓语要用过去完成时)
He had hardly arrived when it began to snow. 他一到，天就下起雪来了。(hardly…when 的主句谓语要用过去完成时)
