This document discusses the four types of conditional sentences in English: zero, first, second, and third conditional. It provides examples of each type and explains their uses. The zero conditional describes simple cause and effect or scientific truths. The first conditional expresses likelihood or promises/threats. The second conditional discusses unlikely or imaginary situations. The third conditional talks about impossible past situations or expresses regrets. It also covers alternatives to "if" such as "unless", "otherwise", and mixed conditionals.