In modern linguistics, parts of speech are discriminated according to three criteria: semantic, formal and functional. This approach may be defined as complex. The semantic criterion presupposes the grammatical meaning of the whole class of words (general grammatical meaning). The formalcriterion reveals paradigmatic properties: relevant grammatical categories, the form of the words, their specific inflectional and derivational features. The functionalcriterion concerns the syntactic function of words in the sentence and their combinability. Thus, when characterizing any part of speech we are to describe: a) its semantics; b) its morphological features; c) its syntactic peculiarities.
In accordance with the said criteria, we can classify the words of the English language into notional and functional.