The document discusses the use of "have to" and related expressions in different contexts and tenses. It explains that "have to" expresses obligation or necessity, while "don't have to" means there is no obligation. In the past, "had to" is used instead of "have to" to talk about past obligations. Similarly, "will have to" refers to future obligations. The negative forms of obligation are also discussed. Examples are provided to illustrate the appropriate uses of "have to", "don't have to", "had to", "will have to", and related expressions in different situations.