Theoretical Grammaris a section of linguistics that studies grammar system of language.
Grammar system of language refers to the whole complex of conformities to natural laws where the latter defines ways of words’ alterations and also ways of word combinations in phrases and sentences.
As any complex object Grammar is a complex system that is presented by elements and structure in their mutually dependent organization.
Grammar elements refer to morphemes, words, word-combinations and sentences.
Grammar structure implies relations and connections among grammar elements or inner organization of the language grammar system.
The subjectof English Theoretical Grammar refers to the study of the English Language grammar organization as a system parts of which are mutually connected with definite relations of different types of complexity (complication, complicacy).
The main taskof Theoretical Grammar is an adequate systematic (methodic) description of language facts and also their theoretical interpretation.
The difference between Practical and Theoretical Grammar refers to the following peculiarities:
1) Practical Grammar prescribes definite rules for the use of a language (gives instructions for the use of language data, teaches how to speak and write);
2) Theoretical Grammar analyzes language data, interprets that, expounds the data but does not give instructions as for the use of them.