The sentences that include a gerund are:
- "While swimming at the beach, Harper found a starfish." (The gerund is "swimming.")
- "Swimming in the gulf is Finn's favorite summer activity." (The gerund is "swimming.")
- "Nash has been swimming since he was five years old." (The gerund is "swimming.")
Among these, the best example that clearly illustrates the use of a gerund as a subject is "Swimming in the gulf is Finn's favorite summer activity."