1. Tense Categories of the English Language

11. Phraseological units, or idioms and free word group
Idioms - structurally, lexically and semantically fixed phrases or sentences having mostly the meaning, which is not made up by the sum of meanings of their component parts.
Theyare characterised by a double sense: the current meanings of constituent words build up a certain picture, but the actual meaning of the whole unit has little or nothing to do with that picture, in itself creating an entirely new image.
All PU are divided into 3 groups:
Ph. Fusions – non-motivated and indivisible. To show white feather – струсить;
Ph. Unities – are motivated, but metaphorical. Monkey business – дурачества;
Ph. Collocations – the meaning of it’s components resulted in the meaning of Ph.unit.
There are two major criteria for distinguishing between phraseological units and free word-groups: semantic and structural.
Academician V. V. Vinogradov spoke, that phraseological units' meaning does not consist of the 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". This phraseological unit is characterised by semantic unity, it conveys a single concept.
The structural criterion also brings distinctive features of phraseological units and free word-groups.
Structural integrity, that is an essential feature of phraseological units, finds expression in a number of restrictions.
First of all, restriction in substitution. As a rule, no word can be substituted in phraseological unit without destroying its sense. To carry coals to Manchester makes as little sense as B Харьков со своим самоваром.
At the same time, in free word-groups substitution does not present any dangers
The second type of restriction is the restriction in additional components.
We can say: This big ship is carrying a large cargo of coal to the port of Liverpool.