过去完成时
过去完成时由助动词had加动词的过去分词构成，主要表示过去某时刻前业已完成的动作或存在的状态，其实就是指过去的过去。使用时往往要要在从句中或有一个明确的过去时间状语。
Before daybreak they had covered half the distance. 拂晓前他们已经走完了大半路程。
No sooner had the rain stopped than they set out again. 雨一停他们就又上路了。
He had not learned any English before he came to the university. 上大学前他一点英语没学过。