Here's why:
* Modals: These are auxiliary verbs (helping verbs) that express possibility, necessity, obligation, or permission. Examples include *can, could, will, would, may, might, must, should, ought to*.
* Helping Verbs: These verbs precede the main verb in a sentence and assist in conveying tense, mood, or voice.
* Nouns: These words represent people, places, things, or ideas.
Nouns do not function as helping verbs or modals.
It's possible you're thinking of a different grammatical concept.
If you could provide more context or an example of where you encountered this term, I might be able to help further.