The document discusses the third conditional, which is used to talk about unreal or hypothetical situations in the past. It has two clauses: an "if" clause in the past perfect tense, and a main clause with "would have" or "could have" plus a perfect modal verb. Some examples are provided such as "If you had driven more carefully, you would not have had an accident" and "If it had not been so crowded, we would have got in."