The document discusses the past perfect continuous tense, including its structure and examples of its use. It is used (1) to indicate an action that was ongoing before a point in the past and (2) to indicate the cause of something that happened in the past. Some examples are "I had been swimming when I fell" and "Sam gained weight because he had been eating too much." The document also notes when to use the past perfect continuous versus the present perfect and encourages using contractions when speaking in the past perfect continuous.