Conditional sentences are used to talk about hypothetical or uncertain situations. There are three types of conditional sentences: Type I refer to possible present or future situations, Type II refer to unlikely or improbable present or future situations, and Type III refer to hypothetical past situations. The document provides examples of conditional sentences for each type, explaining how the tenses are structured in the if-clause and main clause for each. It also discusses the nature and time frames referred to by each type of conditional sentence.