Morphological and syntactico-distributional classifications of words into parts of speech (H.Sweet, O.Jespersen, Ch. Fries.)
Alongside of the three criteria principle of dividing words into grammatical classes there are classifications based on one principle, morphological or syntactic.
The founder of English scientific grammar H.Sweet finds the following classes of words: noun-words, including some pronouns and numerals; adjective-words, including pronouns and numerals; verbs and particles. O.Jespersen names substantives, adjectives, verbs, pronouns and particles. In both cases the term particles denotes words of different classes which have no categories.
The opposite criterion, distributional, is used by the American scholar Ch. Fries. Each class of words is characterized by a set of positions in the sentence, which are defined by substitution testing.
As a result of distributional analysis Ch.Fries singles out four main classes of words, roughly corresponding to nouns, verbs, adjectives and adverbs, and 15 classes of function words.
Notional and functional classes of words.
Notional parts of speech are open classes - new items can be added to them, they are indefinitely extendable. Functional parts of speech are closed systems, including a limited number of members. They cannot be extended by creating new items.
The main notional parts of speech are nouns, verbs, adjectives and adverbs. Members of these four classes are often connected by derivational relations: strength - strengthen.
Functional parts of speech are prepositions, conjunctions, articles, particles. The distinctive features of functional parts of speech are: 1) very general and weak lexical meaning; 2) obligatory combinability; 3) the function of linking and specifying words.
Pronouns constitute a class of words which takes an intermediary position between notional and functional words. On the one hand, they can substitute for nouns and adjectives, on the other hand, pronouns are used as connectives and specifiers. There may be also groups of closed-system items within an open class (notional, functional and auxiliary verbs).
A word in English is very often not marked morphologically and it is easy for words to pass from one class to another (round as a noun, adjective, verb, preposition). Such words are treated either as lexico grammatical homonyms or as words belonging to one class.