What are gerunds?

Gerunds are v-ing forms (FORM) which fill in the slots typically occupied by NPs (e.g., subject, object, complement).

Examples:

1. Swimming is fun. V-ing: Subj. -> gerund

2. I like swimming. V-ing: DO -> gerund

3. I never gave swimming all that much effort. V-ing: IO -> gerund

4. Having practiced my talk gave me more confidence. V-ing (perfect aspect) preceding an NP (my talks)

Notice that in the sentences above, the gerunds fill in the subject, DO, and IO slots which are typically performed by NPs.