Since gerunds function the same way as nouns, they can be used as subjects, direct object, indirect object, or complement. In the same way that nouns can never function as verbs, a gerund can never serve as the main verb, too. If you use a word in the -ing form, it's no longer a gerund, it's called the present participle. What's the difference between gerunds and present participles?
Gerund: Reading books calm me.
Present Participle: She is reading books assigned to her by the teacher.
