The different classes of words – for example nouns, verbs, adjectives, adverbs, prepositions – were traditionally defined by what they denoted – people, places and things (nouns), actions (verbs) and properties (adjectives). This definition is inadequate, although meaning still has an important part to play. Reliable definitions are based on formal criteria – morphological (does a given word take inflectional suffixes or is it invariable?), morpho-syntactic (does a given word take suffixes having to do with person and number, or with case?) and syntactic (where in a phrase or clause does a given word occur?). Once formal criteria have been set up, the connection between word classes and meaning can be studied. This bears not just on what a given word or class of words refers to but also on what speakers and writers do with it – refer, predicate or assert, modify.
