核心规则
过去完成时由 had + 过去分词 构成，表示“过去的过去”。它需要一个过去参照点，突出某动作在该点之前完成、产生结果或持续了一段时间。
结构与例句
By the time we arrived, the film had started。She had never seen snow before she moved north。The ground was wet because it had rained。
常见标志包括 by then、by the time、before、after、already、never，但是否使用仍取决于时间关系，而不是看到标志词就机械套用。
使用场景
叙事中可用过去完成时回溯背景，然后回到一般过去时推进主线。若 before/after 已清楚表明先后，日常表达有时两件事都用一般过去时，但过去完成时更突出先发生者。
常见错误
错误： Yesterday I had visited the museum. → 只有单一过去时间，应写 I visited the museum yesterday。
错误： When I had arrived, they had left. → 若 arrive 是后发生的参照事件，常写 When I arrived, they had left。
易混知识点
现在完成时连接过去与现在；过去完成时连接更早过去与另一个过去点，不能与明确的单一过去时间随意混用。
总结
先画出两个过去事件的时间线，只有需要突出先于过去参照点时才使用 had done。
选择性必修 · 过去之前的完成与回溯
高中学习重点
过去完成时常出现在 It was the first time that...、Hardly/Scarcely had...when...、No sooner had...than... 等结构中。后两类把 hardly/no sooner 置于句首时，主句需要部分倒装。
表示原本希望、打算却未实现，可用 had hoped/intended/planned to do：We had hoped to finish earlier, but the test took longer than expected。
小练习
- 填空：It was the first time that she ___ (speak) in public.
- 补全：No sooner ___ we arrived than the meeting began.
- 选择：By 2020, the team completed / had completed three trials.
参考答案
had spoken。had。had completed，2020 是过去参照点。