The basic units of syntax are words. The first question is then what kinds of words (also known
as parts of speech, or lexical categories, or grammatical categories) does English have? Are
they simply noun, verb, adjective, adverb, preposition, and maybe a few others? Most of us
would not be able to come up with simple definitions to explain the categorization of words.
For instance, why do we categorize book as a noun, but kick as a verb? To make it more difficult,
how do we know that virtue is a noun, that without is a preposition, and that well is an adverb
(in one meaning)?
Words can be classified into different lexical categories according to three criteria: meaning,