The gerund always ends in ing.
- Swimming is my hobby
- Writing is more difficult than reading
However, it should be noted that the present participle also ends in ing.
- They were watching TV when I arrived.
- John will be teaching in that school for the next two years.
You can differentiate between the gerund and the present participle as follows:
- Gerunds function as nouns.
- Gerunds can be a subject, direct objects, indirect objects, and objects of prepositions.
- Present participles, on the other hand, complete progressive verbs or act as modifiers.
Examples of gerund:
- Cycling is fun. (subject)
- I like cycling. (direct object)
- He gives fishing all his time.( indirect object)
- He has devoted all his life to fishing. (object of the preposition to)
Examples of present participle:
- They were eating when the bell rang. (eating is a present participle which is part of the past progressive tense "were eating")
- I bought a fishing book. (writing is a present participle modifying book)