This document discusses the third conditional, a grammatical structure used to talk about hypothetical or imaginary situations in the past and their consequences. The third conditional is formed using "if" plus the past perfect tense (had + past participle) followed by "would" and the perfect infinitive (have + past participle) to discuss something that did not actually happen in the past. An example is provided to illustrate this structure: "If I had known about the meeting, I would have gone."