A ph-l unit can be defined as a reproduced and idiomatic (non-motivated) or partially motivated unit built up according to the model of free word-groups (or sentences) and semantically and syntactically brought into correlation with words.
Structural Criterion.A feature proper both to free phrases and phraseological units is the divisibility of their structure, they consist of separate structural elements. Like in word-groups in phraseological units potentially any component may be changed grammatically, but these changes are rather few, for a stylistic effect.The principal difference between phraseological units and free word-groups manifests itself in the structural invariability of the former. The structural invariability suggests no substitutions of components. to give somebody the cold shoulder means to treat somebody coldly, but a warm shoulder or a cold elbow makes no sense. There are also strict restrictions on the componental extension and grammatical changes of components of phraseological units. The use of the words big. great in a white elephant meaning ‘an expensive but useless thing’ can change or even destroy the meaning of the phraseological unit. The same is true if the plural form feet in the phraseological unit from head to foot is used instead of the singular form. In a free word-group all these changes are possible.
The semantic criterion.The meaning in phraseological units is created by mutual interaction of elements and conveys a single concept. The actual meaning of a phraseological unit is figurative. The transference of the initial word-group can be based on simile, metaphor, metonymy.In the formation of the semantic structure of phraseological units a cultural component plays a special and very important role. The ph. unit red tape originates in the old custom of Government officials and lawyers tying up their papers with red tape. The meaning in a word-group is based on the combined meaning of the words constituting its structure. Each element in a word- combination has a much greater semantic independence and stands for a separate concept, to cur bread, to cut cheese, to eat bread. Every word in a free phrase can form additional syntactic ties with other words outside the expression retaining its individual meaning.
The syntactic criterion. Like word-combinations, phraseological units may have different syntactic functions in the sentence, e.g. the subject (narrow escape, first night), the predicate (to have a good mind, to play Russian roulette, to make a virtue of necessity), an attribute (high and mighty, as ugly as sin), an adverbial (in full swing, on second thoughts. off the record). In accordance with the function they perform in the sentence phraseological units can be classified into: substantive, verbal, adjectival, adverbial, interjectional.
Like free word-groups phraseological units can be divided into coordinative (e.g. the life and soul of something, free and easy, neck and crop) and subordinate (e.g. long in the tooth, a big fish in a little pond, the villain of the piece).
Thus, the characteristic features of phraseological units are: readymade reproduction, structural divisibility, morphological stability, permanence of lexical composition, semantic unity, syntactic fixity.