The document discusses the past perfect tense and past perfect continuous tense in English grammar. The past perfect tense is used to refer to an action that was completed before another past action. It is formed using "had" plus the past participle of the main verb. The past perfect continuous tense refers to an action that was ongoing up until another past action interrupted it. It is formed using "had been" plus the present participle ("-ing" form) of the main verb. Examples are provided to illustrate the usage of each tense.