The words of language, depending on various formal and semantic features, are divided into grammatically relevant classes. The traditional grammatical classes of words are called “parts of speech.”
The problem of parts of speech caused much difficulty both in general linguistics and in the analysis of separate languages. Though it has been studied for more than 200 years, the criteria for defining parts of speech have not been worked out yet. Traditionally grammar gave a semantic definition of parts of speech, taking into account only meaning. However, only meaning cannot be a reliable criterion for defining parts of speech because different parts of speech may have the same meaning and vice versa. E.g. the nouns “books”, “tables”, “students”, denote objects and there are nouns as flight”, “movement”, “arrival”, which do not denote objects but belong to nouns. We see that meaning cannot be the only criterion for defining parts of speech. The structural school of linguistics does not take into account meaning only but only form. Form alone cannot be a reliable criterion either because many parts of speech especially in English may have the same form, e.g. water-to water, silk (adj.) - to silk. Moreover, if we take into account only form, then such unchangeable words as article, particle should be referred to only part of speech.
We see that the criterion of form is not sufficient. The grammatical criterion should be taken into account to give an adequate definition of any part of speech. By grammatical features we mean:
morphological
syntactical
By morphological features different categories are meant. The morphological categories of noun are the categories of number and case. By morphological categories of adjectives we mean the category of quality (degrees of comparison).
By syntactical features of the part of speech the syntactical functions of it are meant. The syntactical function is the most reliable criterion. Thus, the modern conception and amended definition of part of speech should take into account all the above mentioned criteria in complex.
The Problem of Notional and Functional (Formal) Parts of Speech
According to the view held by some grammarians, words should be divided into 2 categories on the following principle: some words denote objects of reality (these would be notional words), whereas other words denote relations are connections between the notional words (these would be formal words or form words).
Notional words are defined as follows: they have full lexical meaning and independent syntactical functions in the sentence. To the notional parts of speech of the English language belong the noun, the adjective, the numeral, the pronoun, the verb, the adverb.
Formal parts of speech differ from notional ones semantically, they are words of incomplete nominative meaning and non-self-dependent, their lexical meaning is more general. Formal parts of speech serve either to express different relations between notional words, they have mediatory functions in the sentence, or they specify the meaning of the word. To the basic functional parts of speech in English belong the article, the preposition, the conjunction, the particle, the modal word, the interjection.
Some linguists are of the opinion that the distinction between notional and formal parts of speech is to some extent relative. The matter is that some words belonging to particular part of speech is occasionally perform a function differing from that which characterized the parts of speech as a whole. E.g., in the sentence “I have some money left” the verb “have” performs the function of the predicate, which is the usual function of a verb in a sentence. In this case it is a notional word. But in the sentence “I have found my briefcase” the verb “have” is an auxiliary; it is a means of forming a certain analytical form of the verb find. So, we have one and the same verb “have” but with different functions on the sentence.
