The two major families of word classes are (1) lexical (or open) classes (nouns, verbs, adjectives, adverbs) and (2) function (or closed) classes (determiners, particles, prepositions, and others). See Examples and Observations, below.
Examples and Observations:
- "When linguists began to look closely at English grammatical structure in the 1940s and 1950s, they encountered so many problems of identification and definition that the term part of speech soon fell out of favour, word class being introduced instead. Word classes are equivalent to parts of speech, but defined according to strict linguistic criteria."
(David Crystal, The Cambridge Encyclopedia of the English Language, 2nd ed. Cambridge Univ. Press, 2003)
- "[The] distinction between lexical and grammatical meaning determines the first division in our classification: form-class words and structure-class words. In general, the form classes provide the primary lexical content; the structure classes explain the grammatical or structural relationship. We can think of the form-class words as the bricks of the language and the structure words as the mortar that holds them together.
(Martha Kolln and Robert Funk, Understanding English Grammar. Allyn and Bacon, 1998)
- One Word, Multiple Classes
"Items may belong to more than one class. In most instances, we can only assign a word to a word class when we encounter it in context. Looks is a verb in 'It looks good,' but a noun in 'She has good looks'; that is a conjunction in 'I know that they are abroad,' but a pronoun in 'I know that' and a determiner in 'I know that man'; one is a generic pronoun in 'One must be careful not to offend them,' but a numeral in 'Give me one good reason.'"
(Sidney Greenbaum, Oxford English Grammar. Oxford Univ. Press, 1996)
- "We recognise the class of a word by its use in context. Some words have suffixes (endings added to words to form new words) that help to signal the class they belong to. These suffixes are not necessarily sufficient in themselves to identify the class of a word. For example, -ly is a typical suffix for adverbs (slowly, proudly), but we also find this suffix in adjectives: cowardly, homely, manly. And we can sometimes convert words from one class to another even though they have suffixes that are typical of their original class: an engineer, to engineer; a negative response, a negative."
(Sidney Greenbaum and Gerald Nelson, An Introduction to English Grammar, 3rd ed. Pearson, 2009)
- A Matter of Degree
"[N]ot all the members of a class will necessarily have all the identifying properties. Membership of a particular class is really a matter of degree. In this regard, grammar is not so different from the real world. There are prototypical sports like 'football' and not so sporty sports like 'darts.' There are exemplary mammals like 'dogs' and freakish ones like the 'platypus.' Similarly, there are good examples of verbs like watch and lousy examples like beware; exemplary nouns like chair that display all the features of a typical noun and some not so good ones like Kenny."
(Kersti Börjars and Kate Burridge, Introducing English Grammar, 2nd ed. Hodder, 2010)