Words fall into lexical categories which determine what affixes they can take.

Verb: verb can take affixes (e.g., : /-ɪŋ/(in progress), /-s/(third-person singular present), /-əd/(past))
Noun: can take affixes (e.g., /-laɪk/(similar to), /-s/(plural), /-s/(possessive))
Adjective: can take affixes. (e.g., /-nɛs/(the property of), /-əst/(most))
Adverbs: The morpheme /-li/ transforms an adjective into an adverb. (e.g., happily, quickly)
Prepositions: (e.g., of, for, to)
Determiners: (e.g., the, this, that, a, your, my)
Pronouns: (e.g., he, she, it, they, them)
Conjunctions: (e.g., and, or, but)

Open Categories (Content Words)

Lexical categories that often allow new words (nouns, verbs, adjectives, adverb).

Closed Categories

Lexical categories that very rarely allow new words (preposition, determiners, pronouns, conjunctions).