听力原文： In recent years, linguists(语言学家) and language teachers have begun to describe language according not only to grammatical structure, using traditional concepts and terms such as the imperative, but also by function. For example, add 'please' to the order 'Open the window!', and the structure remains imperative. The function of the utterance, however, is now that of polite request. Function, then, is determined partly by the user's attitude, and is realized by a variety of linguistic and paralinguistic features—vocabulary, grammar and pronunciation, tone of voice, gesture and facial expression. The language learner can often tell the difference easily between certain basic functions—those of command and request, for example. However, learning to use these different functions of language to communicate effectively is not always so easy. Language function is also determined by the situation in which the language is used. 'Be quiet!' coming from a teacher tired of noisy pupils is obviously an order or command. Used by a nervous bank robber to a fellow in crime, when he thinks he has heard a noise which could have been made by a suspicious policeman, the sentence would be a warning. When we look at the imperative in this way, then, we realize that it consists of a small set of structural forms which can fulfill many different functions: commands, prohibitions, warnings, as well as many others. (30)