Учебно-методическое пособие для студентов математического факультета специальности 1-020501 «Информатика. Иностранный язык (английский)»

Grammatical classes of words 
The words of language, depending on various formal and semantic 
features, are divided into grammatically relevant sets or classes. The traditional 
grammatical classes of words are called «parts of speech». Since the word is 
distinguished not only by grammatical, but also by semantico-lexemic 
properties, some scholars refer to parts of speech as «lexico-grammatical» series 
of words, or as «lexico-grammatical categories». 
It should be noted that the term «part of speech» is purely traditional and 
conventional, it cannot be taken as in any way defining or explanatory. This 
name was introduced in the grammatical teaching of Ancient Greece, where the 
concept of the sentence was not yet explicitly identified in distinction to the 
general idea of speech, and where, consequently, no strict differentiation was 
drawn between the word as a vocabulary unit and the word as a functional 
element of the sentence. 

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In modern linguistics, parts of speech are discriminated on the basis of the 
three criteria: «semantic», «formal», and «functional». The 
semantic 
criterion 
presupposes the evaluation of the generalized meaning, which is characteristic 
of all the subsets of words constituting a given part of speech. This meaning is 
understood as the «categorial meaning of the part of speech». The formal
 
criterion provides for the exposition of the specific inflexional and derivational 
(word-building) features of all the lexemic subsets of a part of speech. The 
functional 
criterion concerns the syntactic role of words in the sentence typical 
of a part of speech. The said three factors of categorial characterization of words 
are conventionally referred to as, respectively, «meaning», «form», and 
«function». 
In accord with the described criteria, words on the upper level of 
classification are divided into notional and functional, which reflects their 
division in the earlier grammatical tradition into changeable and unchangeable. 
To the notional parts of speech of the English language belong the noun, 
the adjective, the numeral, the pronoun, the verb, the adverb. 
(Блох М.Я. Теоретическая грамматика английского языка)