The stock of English words is immense. David Crystal (1995:119) estimates that a conservative figure would approach one million and that, if all the terminology of science were included, it could be twice that much. Each word has a particular role that it can play in the structure of sentences. There are certain grammatical patterns, of sentence and phrase, into which a word may fit, and there may be specific other words with which it may regularly co-occur, its collocational patterning. To describe the grammatical and lexical operation of each of a million words would be a daunting task, and it would turn out that the descriptions would be identical, or very similar, for large sets of words. For this reason, grammarians have traditionally grouped words into classes.
The traditional term has been ‘parts of speech’, but this is not a very transparent term, and linguists now prefer to talk about ‘word classes’. There is substantial, though not universal, agreement about which word classes to recognise for English. They comprise: noun, verb, adjective, adverb, pronoun, determiner, preposition, conjunction. The unfamiliar member of this list is likely to be ‘determiner’, which we will discuss in more detail below. A list of traditional parts of speech would have included an ‘interjection’ class (for items like hey! and cor!), but this is no longer thought necessary; such items would be described as phenomena of spoken discourse.
The word class with the most members is that of ‘noun’, followed by ‘verb’ and ‘adjective’. The ‘adverb’ class is also large, but contains a number of subclasses with restricted membership. These four classes are known as the ‘open’ word classes, because new words in the language are added to one of these. The other classes—pronoun, determiner, preposition, conjunction—have a relatively small membership, which is rarely added to. They are, therefore, known as ‘closed’ classes.
In general, the members of the open (also known as ‘lexical’) word classes provide the main referential (lexical) meaning of a sentence, while the members of the closed (also known as ‘grammatical’) word classes tend to have a structuring function in sentences. This is a gross generalisation; there is more of a spectrum of function from the highly lexical of most nouns and verbs, to the highly grammatical of some determiners, but with the members of some word classes having both a lexical and a grammatical function (e.g. prepositions), but sometimes more one than the other.
The class of ‘determiners’ includes a restricted number of words that are used to accompany nouns in noun phrases. It includes, on the one hand, ‘identifiers’, such as the definite and indefinite articles (a/any the), the possessive identifiers (my, our, your, his, her, its, their) and the demonstrative identifiers (this/these, that/those); and on the other, ‘quantifiers’, such as the numerals (one/two/fifty, first/second/fiftieth) and indefinite quantifiers (some, few, several, plenty of, etc.).