The document discusses the past perfect tense. It begins by reviewing the present perfect tense, noting that it uses the auxiliary verb "have" and expresses actions that occurred at unspecified times in the past or began in the past and continue in the present. It then explains that the past perfect uses the past form of "have", which is "had", along with the past participle of the main verb, and that it expresses two past actions where one occurred before the other. Examples are provided that show one action in the past perfect occurring prior to another past action.