Both a gerund and a present participle come from a verb –ing. . But each has a different function. A gerund acts like a noun while a present participle acts like a verb or adjective.
If it is a gerund, it acts like a noun. It can be a subject, an object, the object of a preposition, or a subject complement.
- Swimming is a great sport. [swimming = subject]
- I hate swimming. [swimming = object ]
- I am afraid of swimming. [swimming = object of a preposition]
- My favourite sport is swimming. [swimming = subject complement]
When swimming is a present participle, it is part of a continuous verb tense.
- Right now, Sheila is swimming. [is swimming = present continuous]
- He was swimming yesterday afternoon. [was swimming = past continuous]
- Tomorrow, my friends and I are going to be swimming. [are going to be swimming = future continuous]
Unlike a gerund, a present participle can act like an adjective that modifies a noun or follows the be verb.
Reading is a relaxing activity ( adj + noun)
Reading is relaxing ( verb to be + adj )
In this case, the word relaxing is a present participle used as an adjective to modify a noun or to follow the verb to be.