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 rules for using the third conditional, such as including continuous forms or modals. Examples are given of exercises to practice the third conditional, including filling in verbs in the correct form and writing new sentences using "if" to describe past situations. Sources for the information are listed at the end.