General Criteria - PoS

characterized by certain grammatical (morphological) forms and a certain grammatical (syntactic) combinability.

The grammatical signs characterize and distinguish between words, rather than their separate forms.

very abstract, common meaning, which is expressed by a combination of grammatical features of these words.

words of the given PoS.
GM: nouns — substantivity, verbs — process, action or state, adjectives — quality, property, numerals — quantity, adverbs — circumstances, quality of the process, etc.

According to the specific inflectional features common to all the words of the gives PoS, i.e. common grammatical (morphological) categories.
GC: nouns — number and case, verbs — tense, mood, voice, aspect, etc., adjectives — degrees of comparison.

According to the syntactic role of the words of the given PoS in the sentence and their ability to combine with other words.
SR: adjectives — attributes, adverbs — adv. modifiers.
C: adjectives — w/nouns, adverbs — w/adjectives and verbs.

and syntactic features play an equally important role. But the syntactic part is more general and that is why it takes the first place in the grammatical characteristic of a word. It is due to the fact that morphologically not all the word classes can be definitely distinguished, but they have their certain syntactic role.
E.g.: interjections and adverbs are both uninflected, but they belong to different PoS due to their different syntactic roles.

the PoS of a given word.
E.g.: -ness, -dom, -hood (nouns), -ate, -ize, -en (verbs), -ly (adverbs), etc.
But the role of these indicators in distinguishing PoS is insignificant and they are applicable only when they do not contradict the results obtained from the grammatical form of the word.
E.g.: difference (n) ‘разница’ — difference (v) ‘отличать’.

any classification must be organized on a constant basis, i.e. the adopted principle(s) must be applicable to all the words of a given class and the classes must be mutually exclusive.
The classification could be consistent if based on a single principle.
A. I. Smirnitsky (1956) doubts the validity of distinguishing PoS according to the formal or functional criteria only: “nouns are nouns not because they are able to decline, but they are able to decline because they are nouns”.
Syntactic role may be induced by the general meaning. E.g.: adjectives — property — combined with nouns — attribute.