THE for grammatical terms

[Xinfu]

-It is gerund that ought to be inflectional.

Is it correct to use the word like this? Forgive me for asking questions so trivial as whether to use a/an/the, because the answer can't be found in dictionaries. Oxford does not say it could be uncountable, but I think it sounds OK as it is. Do you agree?

[Rusty]

It is the gerund that ...

You must use the definite article in this case.
An indefinite article is used in cases when it is not so specific.

The plural gerunds, because gerund is indeed countable, can be used without an article when speaking in general. Otherwise, the definite article is used.

Examples:
A gerund acts as a noun.
Gerunds act as nouns.
Underline the gerunds in this sentence.

[Xinfu]

Excellent answer. Thank you.