The document discusses the different types of conditional sentences in English, including zero, first, second, and third conditionals. It explains the structure and usage of each conditional. The zero conditional uses the present simple tense and expresses general truths. The first conditional refers to present and future possibilities using the present simple and future simple tenses. The second conditional refers to unlikely or imaginary present/future situations using past tense in the if-clause and present conditional in the main clause. The third conditional refers to past situations that were contrary to fact, using past perfect in the if-clause and past conditional in the main clause.