I'm quoting from Essential English Grammar.
The words in bold are gerunds or verbal nouns, i.e. verbs used as nouns.
Running is a good way to keep fit.
Susan is good at
drawing.
I don't like
watching television.
Sometimes it is difficult to know whether an
ing word is a gerund or a present participle.
If you can replace the
ing word or its phrase with the pronoun 'it', then the word is a gerund. Look again at the above sentences and replace the words in bold with 'it'.
I saw him
sitting on the bench.
We cannot say 'I saw him it.'
.