过去完成时由 had + 过去分词 (V3) 构成。它的主要功能是建立 先行性——即它表明一个过去的事件（事件 A）在一个更晚的过去时刻（事件 B，通常用一般过去时表达）发生之前，已经早就完成了（already finished by then）。这个较晚的时刻（事件 B）充当了锚点。
关键规则：过去完成时必须有一个锚点（过去的一个时间点或事件）才能成立。如果没有锚点，只需使用一般过去时即可。
The Past Perfect tense is formed using had + Past Participle (V3). Its primary function is to establish antecedence—meaning it signals that one past event (Event A) was entirely already finished by then (already completed) before a later past moment (Event B), which is typically expressed using the Simple Past. This later moment (Event B) serves as the Anchor Point.
Crucial Rule: You must have an Anchor Point (a time or an event in the past) for the Past Perfect to make sense. Without the anchor, the Simple Past is sufficient.