This document provides a lesson on using past modals to express regret over past actions or situations that did not occur. It discusses examples of companies like Warner Brothers and Procter & Gamble failing to anticipate future innovations like sound in movies and disposable diapers. Modal verbs like could have, should have, must have, and would have are explained as ways to talk about unfulfilled past obligations or possibilities. Specific uses of each modal in the past tense are defined and illustrated with examples.