So, phraseological units are included into the system of parts of speech.
Phraseological units are created from free word-groups. But in the course of time some words constituents of phraseological units may drop out of the language; the situation in which the phraseological unit was formed can be forgotten, motivation can be lost and these phrases become phraseological fusions.
The vocabulary of a language is enriched not only by words, but also by phraseological units. Phraseological units are word-groups that cannot be made in the process of speech, they exist in the language as ready-made units. They are compiled in special dictionaries. The same as words phraseological units express a single notion and are used in a sentence as one part of it. American and British lexicographers call such units «idioms». We can mention such dictionaries as: L. Smith «Words and Idioms», V. Collins «A Book of English Idioms» etc. In these dictionaries we can find words, peculiar in their semantics (idiomatic), side by side with word-groups and sentences. In these dictionaries they are arranged, as a rule, into different semantic groups.
Phraseological units can be classified according to the ways they are formed, according to the degree of the motivation of their meaning, according to their structure and according to their part-of-speech meaning.
A.V.Koonin classified phraseological units according to the way they are formed. He pointed out primary and secondary ways of forming phraseological units.
Among two-top units A.I.Smirnitsky points out the following structural types:
a) attributive-nominal such as: a month of Sundays, grey matter, a millstone round ones neck and many others. Units of this type are noun equivalents and can be partly or perfectly idiomatic. In partly idiomatic units (phrasisms) sometimes the first component is idiomatic, e.g. high road, in other cases the second component is idiomatic, e.g. first night. In many cases both components are idiomatic, e.g. red tape, blind alley, bed of nail, shot in the arm and many others.
b) verb-nominal phraseological units, e.g. to read between the lines, to speak BBC, to sweep under the carpet etc. The grammar centre of such units is the verb, the semantic centre in many cases is the nominal component, e.g. to fall in love. In some units the verb is both the grammar and the semantic centre, e.g. not to know the ropes. These units can be perfectly idiomatic as well, e.g. to burn ones boats, to vote with ones feet, to take to the cleaners etc.
Very close to such units are word-groups of the type to have a glance, to have a smoke. These units are not idiomatic and are treated in grammar as a special syntactical combination, a kind of aspect.
c) phraseological repetitions, such as: now or never, part and parcel, country and western etc. Such units can be built on antonyms, e.g. ups and downs, back and forth; often they are formed by means of alliteration, e.g cakes and ale, as busy as a bee. Components in repetitions are joined by means of conjunctions. These units are equivalents of adverbs or adjectives and have no grammar centre. They can also be partly or perfectly idiomatic, e.g. cool as a cucumber (partly), bread and butter (perfectly).
Phraseological units the same as compound words can have more than two tops (stems in compound words), e.g. to take a back seat, a peg to hang a thing on, lock, stock and barrel, to be a shadow of ones own self, at ones own sweet will.
Phraseological units can be classified as parts of speech. This classification was suggested by I.V.Arnold. Here we have the following groups:
a) noun phraseologisms denoting an object, a person, a living being, e.g. bullet train, latchkey child, redbrick university, Green Berets,
b) verb phraseologisms denoting an action, a state, a feeling, e.g. to break the log-jam, to get on somebodys coattails, to be on the beam, to nose out, to make headlines,
c) adjective phraseologisms denoting a quality, e.g. loose as a goose, dull as lead
d) adverb phraseological units, such as: with a bump, in the soup, like a dream, like a dog with two tails,
e) preposition phraseological units, e.g. in the course of, on the stroke of,
f) interjection phraseological units, e.g. «Catch me!», «Well, I never!» etc.
InI.V. Arnolds classification there are also sentence equivalents, proverbs, sayings and quotations, e.g. «The sky is the limit», «What makes him tick», «I am easy». Proverbs are usually metaphorical, e.g. «Too many cooks spoil the broth», while sayings are as a rule non-metaphorical, e.g. «Where there is a will there is a way».