Gerunds are used in any place where you could use a regular noun. You take the verb form, add "ing" and use it as a noun (as the subject the predicate nominative, the direct object, the object of preposition, or any other noun function). For instance, "READING is my favorite hobby." Here, the gerund, reading, is acting as the subject of the sentence. I love JOGGING. What do I love? Jogging is the direct object of this sentence. Of course, gerunds can also have other words attached to them (a gerund phrase), but getting a good understanding of a simple gerund is a must before moving on.