This document discusses the use of "have to" to express necessity or lack of necessity. It provides examples of affirmative sentences using "have to/has to" to mean something is necessary, such as "I have to study for my English test." Negative sentences using "don't have to/doesn't have to" indicate something is not necessary, like "You don't have to do that." Interrogative sentences ask if something is necessary, for example "Do I have to clean it?" The document provides forms and examples of using "have to" in the affirmative, negative, and interrogative to express if something is obligatory or optional.