This document discusses the past perfect tense in English. The past perfect, also called the past of the past, is used to talk about something that happened before another action in the past. It discusses the structure of the past perfect, which uses the auxiliary verb "had" plus the past participle of the main verb. Examples are provided to illustrate how the past perfect is used with adverbs of time like "when" and "by the time" to show that one past action occurred before another in the past. Question forms of the past perfect are also explained.