The document discusses the past perfect tense in English. It is used to describe actions that were completed before something else in the past. There are two main uses - to indicate an action that was completed before another past action, and to indicate a past duration up until another past time. Examples are provided for each use. The formation of the past perfect tense is also explained, including its affirmative and negative forms. Practice exercises are provided to help the reader better understand when to use the past perfect tense.