The "-ing" form, known as a gerund, is created by adding "-ing" to a verb. It functions as a noun in a sentence, allowing actions to be discussed as concepts or activities. For example, in the sentence "Swimming is fun," the word swimming acts as the subject. Gerunds can also be used after certain verbs, prepositions, or adjectives. For instance, in "I enjoy reading books," the gerund reading follows the verb "enjoy." This versatility makes gerunds useful for expressing actions in various grammatical contexts.