The word function is often used in different senses in the literature of linguistics. In traditional grammar, the word function is used in the sense of “organic function”, namely, what part of speech can serve as what sentence element. In sociological studies of language, function is often used to refer to the role language plays in society. In ethnography, scholars use function to refer to the specific uses of language. In systemic-functional linguistics, functions of language are highly generalized categories of meaning which simultaneously underlie an utterance. According to Halliday (1972), there are three metafunctions:

·Ideational function --- we use language to talk about our experience of the world, including our inner world, to describe events, states and the entities involved (language serves as a cording system which deals with the relation between man and nature);

·Interpersonal function --- we use language to interact with others, to establish and maintain relations with them, to please them, to anger them, and influence their behavior, to get their help or sympathy (language servers as a medium between individuals);

·Textual function --- language as a system organizes messages in a unified manner so that chunks of messages fit logically with others around them and with the wider context in which the talking or writing takes place (when language is in use, playing the above two functions, it naturally forms a text).

　 These metafunctions of language account for why language is as it is, a point which will be made clear in the following three sections.