GERUNDS

can turn -ing verbs into nouns

can be used as a subject, a noun, or an object of a preposition

Working hard is the secret to success. (subject)

I love watching the sunrise. (direct object)

The secret to success is working hard. (predicate nominative)

Good grades are the result of studying. (object of preposition)

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A noun is best defined as a word that can be the subject, object or indirect object of a sentence, and a gerund and an infinitive can both be those things.

Running is hard.

I gave running another go.

I chose running.

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The tricky thing with the gerund is that it looks exactly the same as the present participle and the present continuous tense, which can cause a bit of confusion.

I am running late. (Present continuous)

I am the running man. (Present participle used as adjective)

As a gerund is a verbal noun and not a normalised noun, it still retains some verbal behaviours, such as taking an adverb as a modifier.

Running slowly is hard.

And it can take an object, though it is not the sentence object but rather the object of the noun phrase in which the gerund is the head.

Walking the dog is hard work. (gerund + object)

compare to:

I am walking the dog. (present continuous)