There are two types of main verbs: transitive and intransitive. A transitive verb expresses an action that has an object, such as "write a letter" or "buy a car." An intransitive verb expresses an action without an object, such as "ran fast" or "went to school." The document provides examples of sentences using transitive and intransitive verbs and explains that transitive verbs can be changed to passive voice while intransitive verbs cannot.