The document summarizes the structure and use of the third conditional. It is used to talk about unreal past situations or express regrets. The structure is "If + subject + past perfect + subject + would/could/might + have + past participle". Examples are provided such as "If I had studied harder, I would have passed the exam" and "If I had known, I wouldn't have done that". The document then provides pictures for an activity to make third conditional sentences about the pictures, such as "If he hadn’t stolen, he wouldn’t have got into jail".