Main verbs are the primary action words in a sentence that express what the subject is doing. They can stand alone or be accompanied by helping verbs. For example, in the sentence "She runs every morning," the word runs is the main verb that indicates the action performed by the subject. Main verbs can be categorized into two types: action verbs and linking verbs. Action verbs describe physical or mental activities, like jump or think, while linking verbs connect the subject to a subject complement, often describing a state of being, such as is or seem.