The above two sections discuss what language is and what are the characteristics of human languages. To understand what language is we will naturally think about what language does. Function is the term used to discuss the question, but the word is often used in different senses in the literature.

　 The word function is used by some grammarians to refer to organic or constitutive functions of linguistic units. In this case, function means “a unit's ‘privilege of occurrence’, in terms of its position, mobility, optionality, etc, in the unit of which it is a constituent.” (Quirk et al 1985: 48) Units which may appear in the same slot in the sentence have the same syntactic function.

　(1) It has been very cold recently.
　(2) It has been very cold this month.
　(3) It has been very cold during the past two weeks.

　 The italicized parts of (1), (2), (3) are functionally equivalent, for the substitution does not harm the grammaticality of the sentences.

　‘Function’ is generally used in linguistics to refer to the roles language plays in our life or in society. Even in this sense, ‘function’ may refer to specific roles or general roles.
The specific roles language plays fulfill an individual's purpose of communication. Speech functions may be an accurate term for this kind of roles. When someone steps on an English speaker's toes, he/she will utter ‘Ouch!’ almost instinctively. Football fans sometimes cannot help, during the match, shouting to cheer the team, or even swearing at a player or the umpire. In these and many other similar cases, language helps to relieve the nervous/physical energy. It plays an emotive or expressive function. Linguists have tried to generalize speech functions and classify them into several types. But the list is hardly exhaustive.

　 The general roles language plays are termed metafunctions. A metafunction is a more abstract one, which is capable of describing innumerable specific functions. According to M.A.K. Halliday (1970), language plays three metafunctions simultaneously.

　(i) The ideational function

　(ii) The interpersonal function

　(iii) The textual function