How to Distinguish Phraseological Units from Free Word-Groups
This is probably the most discussed — and the most controversial — problem in the field of phraseology. The task of distinguishing between free word-groups and phraseological units is further complicated by the existence of a great number of marginal cases, the so-called semi-fixed or semi-free word-groups, also called non-phraseological word-groups which share with phraseological units their structural stability but lack their semantic unity and figurativeness (e. g. to go to school, to go by bus, to commit suicide).
There are two major criteria for distinguishing between phraseological units and free word-groups: semantic and structural.
Compare the following examples:
A. Cambridge don: I'm told they're inviting more American professors to this university. Isn't it rather carrying coals to Newcastle?
(To carry coals to Newcastle means "to take something to a place where it is already plentiful and not needed". Cf. with the R. В Тулу со своим самоваром.)
В. This cargo ship is carrying coal to Liverpool.
The first thing that captures the eye is the semantic difference of the two word-groups consisting of the same essential constituents. In the second sentence the free word-group is carrying coal is used in the direct sense, the word coal standing for real hard, black coal and carry for the plain process of taking something from one place to another. The first context quite obviously has nothing to do either with coal or with transporting it, and the meaning of the whole word-group is something entirely new and far removed from the current meanings of the constituents.
Academician V. V. Vinogradov spoke of the semantic change in phraseological units as "a meaning resulting from a peculiar chemical combination of words". This seems a very apt comparison because in both cases between which the parallel is drawn an entirely new quality comes into existence.
The semantic shift affecting phraseological units does not consist in a mere change of meanings of each separate constituent part of the unit. The meanings of the constituents merge to produce an entirely new meaning: e. g. to have a bee in one's bonnet means "to have an obsession about something; to be eccentric or even a little mad". The humorous metaphoric comparison with a person who is distracted by a bee continually buzzing under his cap has become erased and half-forgotten, and the speakers using the expression hardly think of bees or bonnets but accept it in its transferred sense: "obsessed, eccentric".
That is what is meant when phraseological units are said to be characterized by semantic unity. In the traditional approach, phraseological units have been defined as word-groups conveying a single concept (whereas in free word-groups each meaningful component stands for a separate concept).
It is this feature that makes phraseological units similar to words: both words and phraseological units possess semantic unity (see Introduction). Yet, words are also characterized by structural unity which phraseological units very obviously lack being combinations of words.
Most Russian scholars today accept the semantic criterion of distinguishing phraseological units from free word-groups as the major one and base their research work in the field of phraseology on the definition of a phraseological unit offered by Professor A. V. Koonin, the leading authority on problems of English phraseology in our country:
"A phraseological unit is a stable word-group characterized by a completely or partially transferred meaning."
The definition clearly suggests that the degree of semantic change in a phraseological unit may vary ("completely or partially transferred meaning"). In actual fact the semantic change may affect either the whole word-group or only one of its components. The following phraseological units represent the first case: to skate on thin ice (@ to put oneself in a dangerous position; to take risks); to wear one's heart on one's sleeve1 (@ to expose, so that everyone knows, one's most intimate feelings); to have one's heart in one's boots (@ to be deeply depressed, anxious about something); to have one's heart in one's mouth (@ to be greatly alarmed by what is expected to happen); to have one's heart in the right place (@ to be a good, honest and generous fellow); a crow in borrowed plumes (@ a person pretentiously and unsuitably dressed; cf. with the R. ворона в павлиньих перьях); a wolf in a sheep's clothing1 (@ а dangerous enemy who plausibly poses as a friend).
The second type is represented by phraseological units in which one of the components preserves its current meaning and the other is used in a transferred meaning: to lose (keep) one's temper, to fly into a temper, to fall ill, to fall in love (out of love), to stick to one's word (promise), to arrive at a conclusion, bosom friends, shop talk (also: to talk shop), small talk.
Here, though, we are on dangerous ground because the border-line dividing phraseological units with partially changed meanings from the so-called semi-fixed or non-phraseological word-groups (marginal cases) is uncertain and confusing.
The term "idiom", both in this country and abroad, is mostly applied to phraseological units with completely transferred meanings, that is, to the ones in which the meaning of the whole unit does not correspond to the current meanings of the components. There are many scholars who regard idioms as the essence of phraseology and the major focus of interest in phraseology research.
The structural criterion also brings forth pronounced distinctive features characterizing phraseological units and contrasting them to free word-groups.
Structural invariability is an essential feature of phraseological units, though, as we shall see, some of them possess it to a lesser degree than others. Structural invariability of phraseological units finds expression in a number of restrictions.
First of all, restriction in substitution. As a rule, no word can be substituted for any meaningful component of a phraseological unit without destroying its sense. To carry coals to Manchester makes as little sense as Б Харьков со своим самоваром.
The idiom to give somebody the cold shoulder means "to treat somebody coldly, to ignore or cut him", but a warm shoulder or a cold elbow make no sense at all. The meaning of a bee in smb's bonnet was explained above, but a bee in his hat or cap would sound a silly error in choice of words, one of those absurd slips that people are apt to make when speaking a foreign language.
At the same time, in free word-groups substitution does not present any dangers and does not lead to any serious consequences. In The cargo ship is carrying coal to Liverpool all the components can be changed: The ship/vessel/boat carries/transports/takes/brings coal to (any port).
The second type of restriction is the restriction in introducing any additional components into the structure of a phraseological unit.
In a free word-group such changes can be made without affecting the general meaning of the utterance: This big ship is carrying a large cargo of coal to the port of Liverpool.
In the phraseological unit to carry coals to Newcastle no additional components can be introduced. Nor can one speak about the big white elephant (when using the white elephant in its phraseological sense) or about somebody having his heart in his brown boots.
Yet, such restrictions are less regular. In Vanity Fair by W. M. Thackeray the idiom to build a castle in the air is used in this way:
"While dressing for dinner, she built for herself a most magnificent castle in the air of which she was the mistress ..."
In fiction such variations of idioms created for stylistic purposes are not a rare thing. In oral speech phraseological units mostly preserve their traditional structures and resist the introduction of additional components.
The third type of structural restrictions in phraseological units is grammatical invariability. A typical mistake with students of English is to use the plural form of fault in the phraseological unit to find fault with somebody (e. g. The teacher always found faults with the boy). Though the plural form in this context is logically well-founded, it is a mistake in terms of the grammatical invariability of phraseological units. A similar typical mistake often occurs in the unit from head to foot (e. g. From head to foot he was immaculately dressed). Students are apt to use the plural form of foot in this phrase thus erring once more against the rigidity of structure which is so characteristic of phraseological units.
Yet again, as in the case of restriction in introducing additional components, there are exceptions to the rule, and these are probably even more numerous.
One can build a castle in the air, but also castles. A shameful or dangerous family secret is picturesquely described as a skeleton in the cupboard, the first substantive component being frequently and easily used in the plural form, as in: I'm sure they have skeletons in every cupboard! A black sheep is a disreputable member of a family who, in especially serious cases, may be described as the blackest sheep of the family.
Consider the following examples of proverbs:
We never know the value of water till the well is dry.
You can take the horse to the water, but you cannot make him drink.
Those who live in glass houses shouldn't throw stones.
Even these few examples clearly show that proverbs are different from those phraseological units which have been discussed above. The first distinctive feature that strikes one is the obvious structural dissimilarity. Phraseological units, as we have seen, are a kind of ready-made blocks which fit into the structure of a sentence performing a certain syntactical function, more or less as words do. E. g. George liked her for she never put on airs (predicate). Big bugs like him care nothing about small fry like ourselves, (a) subject, b) prepositional object).
Proverbs, if viewed in their structural aspect, are sentences, and so cannot be used in the way in which phraseological units are used in the above examples.
If one compares proverbs and phraseological units in the semantic aspect, the difference seems to become even more obvious. Proverbs could be best compared with minute fables for, like the latter, they sum up the collective experience of the community. They moralize (Hell is paved with good intentions), give advice (Don't judge a tree by its bark}, give warning (If you sing before breakfast, you will cry before night), admonish (Liars should have good memories), criticize (Everyone calls his own geese swans).
No phraseological unit ever does any of these things. They do not stand for whole statements as proverbs do but for a single concept. Their function in speech is purely nominative (i. e. they denote an object, an act, etc.). The function of proverbs in speech, though, is communicative (i. e. they impart certain information).
The question of whether or not proverbs should be regarded as a subtype of phraseological units and studied together with the phraseology of a language is a controversial one.
Professor A. V. Koonin includes proverbs in his classification of phraseological units and labels them communicative phraseological units (see Ch. 13). From his point of view, one of the main criteria of a phraseological unit is its stability. If the quotient of phraseological stability in a word-group is not below the minimum, it means that we are dealing with a phraseological unit. The structural type — that is, whether the unit is a combination of words or a sentence — is irrelevant.
The criterion of nomination and communication cannot be applied here either, says Professor A. V. Koonin, because there are a considerable number of verbal phraseological units which are word-groups (i. e. nominative units) when the verb is used in the Active Voice, and sentences (i. e. communicative units) when the verb is used in the Passive Voice. E. g. to cross (pass) the Rubicon — the Rubicon is crossed (passed); to shed crocodile tears — crocodile tears are shed. Hence, if one accepts nomination as a criterion of referring or not referring this or that unit to phraseology, one is faced with the absurd conclusion that such word-groups, when with verbs in the Active Voice, are phraseological units and belong to the system of the language, and when with verbs in the Passive Voice, are non-phraseological word-groups and do not belong to the system of the language. 
It may be added, as one more argument in support of this concept, that there does not seem to exist any rigid or permanent border-line between proverbs and phraseological units as the latter rather frequently originate from the former.
So, the phraseological unit the last straw originated from the proverb The last straw breaks the camel's back, the phraseological unit birds o/ a feather from the proverb Birds of a feather flock together, the phraseological unit to catch at a straw (straws) from A drowning man catches at straws.
What is more, some of the proverbs are easily transformed into phraseological units. E. g. Don't put all your eggs in one basket > to put all one's eggs in one basket; don't cast pearls before swine > to cast pearls before swine.