Gerunds and infinitives can both be used as subjects and complements of sentences. A gerund is a verb form ending in "-ing" that functions as a noun, such as "reading" or "smoking." An infinitive is the base verb form preceded by "to", such as "to learn" or "to go." As subjects and complements, gerunds are more commonly used and sound more natural, while infinitives emphasize potential and sound more abstract. When used as objects, the choice depends on the main verb - some verbs take gerund objects and some take infinitive objects.