1.1. Structural Criterion

The structural criterion brings forth pronounced features.

A feature proper both to free phrases and phraseological units is the divisibility (раздельнооформленность) of their structure, i.e. they consist of separate structural elements. In phraseological units potentially any component may be changed grammatically, but these changes are rather few, limited and occasional and usually serve 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 (or rather limited) substitutions of components. For example, to give somebody the cold shoulder means 'to treat somebody coldly, to ignore or cut him', 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.

1.2. Semantic Criterion

The semantic criterion is of great help in stating the semantic difference/similarity between free word-groups and phraseological units, and between phraseological units and words.

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 (transferred) and is opposed to the literal meaning of a word-combination from which it is derived. e.g. to skate on thin ice - 'to take risks’.

In a free phrase the semantic correlative ties are fundamentally different. 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, e.g. to cut bread, to cut cheese, to eat bread. The semantic similarity between the two is proved by the fact that, the meaning is understood as a whole and not related to the meaning of individual words.

1.3. Syntactic Criterion

The syntactic criterion reveals the close ties between single words and phraseological units as well as free word-groups. Like words phraseological units may have different syntactic functions in the sentence. Like free word-groups phraseological units can be divided into coordinative and subordinative. Thus, the characteristic features of phraseological units are: ready-made reproduction, structural divisibility, morphological stability, permanence of lexical composition, semantic unity, syntactic fixity.

4. Classification of phraseological units

According to the degree of idiomaticity phraseological units can be classified into three big groups: phraseological fusions (сращения), phraseological unities (единства) and phraseological collocations (сочетания).

Phraseological fusions are completely non-motivated word-groups, e.g. as mad as a hatter - 'utterly mad'; white elephant - 'an expensive but useless thing'.

Phraseological unities are partially non-motivated as their meaning can usually be perceived through the metaphoric meaning of the whole phraseological unit, e.g. to bend the knee - 'to submit to a stronger force, to obey submissively.

Phraseological collocations are not only motivated but contain one component used in its direct meaning, while the other is used metaphorically, e.g. to meet the requirements, to attain success

According to their origin all phraseological units may be divided into: native and borrowed.

The main sources of native phraseological units are:

1) terminological and professional lexics, e.g. physics: center of gravity (центр тяжести); navigation: cut the painter (обрубить канат) - 'to become independent', military sphere: fall into line (стать в строй) - 'conform with others';

2) British literature, e.g. the green-eyed monster - 'jealousy' (W.Shakespeare); never say die - 'do not give up hope in a difficult situation' (Ch.Dickens);

3) British traditions and customs, e.g. baker's dozen - 'a group of thirteen'. In the past British merchants of bread received from bakers thirteen loaves instead of twelve and the thirteenth loaf was merchants' profit.

4) superstitions and legends, e. g. a black sheep - 'a less successful or more immoral person in a family or a group'. People believed that a black sheep was marked by the devil;

5) historical facts and events, personalities, e.g. to do a Thatcher - 'to stay in power as prime minister for three consecutive terms. (from the former Conservative prime minister Margaret Thatcher)';

6) phenomena and facts of everyday life, e.g. carry coals to Newcastle - 'to take something to a place where there is plenty of it available. Newcastle is a town in Northern England where a lot of coal was produced; to get out of wood - 'to be saved from danger or difficulty'.

The main sources of borrowed phraseological units are:

1) the Holy Script, e.g. the left hand does not know what the right hand is doing - 'communication in an organization is bad so that one part does not know what is happening in another part;

2) ancient legends and myths belonging to different religious or cultural traditions, e.g. to cut the Gordian knot - 'to deal with a difficult problem in a strong, simple and effective way', (Gordius, king of Gordium, tied an intricate knot and prophesied that whoever untied it would become the ruler of Asia. It was cut through with a sword by Alexander the Great);

3) facts and events of the world history, e.g. to cross the Rubicon - 'to do something which will have very important results which cannot be changed after. Julius Caesar started a war which resulted in victory for him by crossing the river Rubicon in Italy; to meet one's Waterloo;

4) variants of the English language, e.g. a heavy hitter - 'someone who is powerful and has achieved a lot' (American);

5) other languages (classical and modern), e.g. second to none - 'equal with any other and better than most' (from Latin); the fair sex - 'women' (from French); let the cat out of the bag — 'reveal a secret carelessly or by mistake' (from German); tilt at windmills — 'to waste time trying to deal with enemies or problems that do no exist' (from Spanish);