• There are eight word classes in English:
• Almost all English words belong to one or another of the eight word classes. A word is placed in a particular class according to how it is used in building phrases: nouns are used in noun phrases, verbs in verb phrases, adjectives in adjective phrases, adverbs in adverb phrases and prepositions in prepositional phrases. Determiners (including the articles “a” and “the”) are used in connection with nouns. Pronouns are used to replace either single nouns or whole noun phrases. Conjunctions are used to connect the clauses that make up compound and complex sentences.
• There is a close connection between the eight word classes and the five clause elements—subjects, verbs (verb elements), objects, complements, and adverbials. The various types of phrase function as one or another of the clause elements; they are the “constituents” of the clause elements in other words. Noun phrases, for example, can function as subjects, objects, or, sometimes, adverbials; either noun phrases or adjective phrases can function as complements; verbs (and only verbs) function as parts of the verb element of a clause.
• The labels used to name the word classes do not always accurately reflect the way the words in the class are used. Adverbs, for example, are used to modify adjectives as well as verbs and pronouns often substitute for a whole noun phrase, not for a single noun.
• Although generally speaking, the members of word classes are single words (written with a space before and after) multi-word semantic units such as phrasal verbs and two-word prepositions are also regarded as members of word classes. (For example “find out” is regarded as a unitary verb and “together with” as a unitary preposition.
• Teaching and terminological notes:
(1) The phrases that function as clause elements frequently are made up of only one word. Such phrases can be referred to as “unitary constituents” of clauses, where “unitary” is to be understood as meaning “containing only one word.“ This terminology can be confusing because there is a tradition of restricting the use of the word “phrase” to groups of two or more words. It is well worth adopting, however, because if single-word phrases were not “allowed” it would be necessary to abandon an idea of enormous explanatory value: that clauses are composed of phrases.
(2) There is a tradition of defining the names of the word classes “notionally” (i.e. in semantic terms.) For example, a noun is often said to be “the name of a person, place or thing.” This practice may be useful in certain teaching situations, but it should always be kept in mind that if even a moderately deep understanding of grammar is to be achieved, grammatical concepts must be defined grammatically.