This document explains the 3rd conditional, which is used to talk about hypothetical situations in the past. It begins by reviewing the 1st and 2nd conditionals. It then shows how to form the 3rd conditional using "if + past perfect, would have + past participle". Examples are provided like "If I had been alive 2000 years ago, I would have been a Roman". The document concludes with exercises transforming sentences into the 3rd conditional form.