The document discusses conditional sentences in English. It explains that conditional sentences are used to talk about unreal or unlikely situations. There are two main types discussed - the second conditional which refers to the present or future but an unlikely situation, and the third conditional which refers to unlikely past situations. Examples are provided of sentence structures for both types, including using "if+past tense" in the condition clause and "would/could/might + infinitive" in the result clause. Various uses of modal verbs, tenses, and word order inversions in conditional sentences are also covered.