Interjections (oh, hey, alas, uh, um), are a smaller open class, that also includes greetings (hello, goodbye), and question responses (yes, no, uh-huh).

University of Michigan - Ann Arbor

Open classes are changed frequently —  new words like iPhone or to fax are continually being created or borrowed. Four major open classes occur in the languages of the world: nouns (including proper nouns), verbs, adjectives, and adverbs, as well as the smaller open class of interjections.

Open class

An on-going but a helpful book resource about NLP
https://web.stanford.edu/~jurafsky/slp3/

Speech and Language Processing (3rd ed. draft) 

Nouns are words for people, places, or things, but include others as well.

Noun

Verbs refer to actions and processes, including main verbs like draw, provide, andgo.

Verb

Adjectives often describe properties or qualities of nouns, like color (white, black), age (old, young), and value (good, bad).

Adjectives

Adverbs generally modify something (often verbs, hence the name “adverb”, but also other adverbs and entire verb phrases).

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