The document discusses various ways to use past tenses to describe unreal or imaginary situations in the present or future. Some of the structures covered include "it's (high/about) time", "what if/suppose/imagine", "would rather/would sooner", "as if/as though", and "I wish/if only". These structures allow us to talk about preferences, hypothetical scenarios, counterfactual situations, and things we want to be different than they are. The document provides examples of how to use each structure and encourages the reader to practice making their own sentences using these past tense constructions.