This document provides a guide on the third conditional, also known as the past or impossible conditional. It describes how the third conditional expresses a hypothetical situation in the past that did not actually occur. It lists some key features: we can use continuous forms or modals, and in US English "would have" can be used in the if-clause. Exercises are provided to practice the third conditional through verb conjugations and writing sample sentences using "if" to describe past hypothetical situations contrary to known facts. Sources for the information are listed.