Functional theories of grammar are those approaches to the study of language that see the functions of language and its elements to be the key to understanding linguistic processes and structures. Functional theories of language propose that since language is fundamentally a tool, it is reasonable to assume that its structures are best analyzed and understood with reference to the functions they carry out. Functional theories of grammar differ from formal theories of grammar, in that the latter seek to define the different elements of language and describe the way they relate to each other as systems of formal rules or operations, whereas the former defines the functions performed by language and then relates these functions to the linguistic elements that carry them out. This means that functional theories of grammar tend to pay attention to the way language is actually used in communicative context, and not just to the formal relations between linguistic elements.
In a broad sense the theories implicit in most work within descriptive linguistics and linguistic typology fit within the category of functional linguistics.
There are several distinct grammatical theories that employ a functional approach.
Dik characterises functional grammar as follows:
In the functional paradigm a language is in the first place conceptualized as an instrument of social interaction among human beings, used with the intention of establishing communicative relationships. Within this paradigm one attempts to reveal the instrumentality of language with respect to what people do and achieve with it in social interaction. A natural language, in other words, is seen as an integrated part of the communicative competence of the natural language user. (2, p. 3)
Because of its emphasis on usage, communicative function, and the social context of language, functional grammar differs significantly from other linguistic theories which stress purely formal approaches to grammar, notably Chomskyan generative grammar. Functional grammar is strongly associated with the school of linguistic typology that takes its lead from the work of Joseph Greenberg.