1) The ability to be modified by adverbs.
2) The ability of the verb to perform the syntactic function of the predicate (this criterion is not absolute).
3) The ability of any verb in the form of the infinitive to combine with a modal verb.
1.2. The Verb: Classifications.
The class of verbs falls into a number of subclasses distinguished by different semantic and lexico-grammatical features. There are identified:
I) On the basis of functional classification:
1) verbs of full nominative value (notional verbs)
This set is derivationally open, it includes the bulk of the verbal lexicon.
2) verbs of partial nominative value (semi-notional verbs and functional verbs).
This set is derivationally closed, it includes limited subsets of verbs. Semi-notional and functional verbs(predicators) serve as markers of predication in the proper sense. They show the connection between the nominative content of the sentence and reality in a specialised way. Among them there are distinguished: