In this article Michael Swan, author of Practical English Usage Third Edition, charts the changes in the English language.

All languages change. The English of 500 years ago is hard for us to understand; the English of 1000 years ago is a foreign language. Even over a much smaller time-scale – ten or twenty years – enough changes take place to make older people complain that the language is going to the dogs, and to force teachers and grammarians to update their descriptions.

Language change happens for various reasons. One powerful mechanism is phonetic erosion. Speech production is a complex physical and mental operation, and speakers naturally seek to economise effort. So unimportant syllables easily lose stress and are reduced, to the point where they may disappear altogether, as in the British pronunciation of words like February ('Febry'), probably ('probly') and secretary ('secretry' or 'secetry'). Small words may be merged in casual speech: have to → 'hafta', want to → 'wanna', got to → 'gotta', going to → 'gonna'; or they can fade away completely: I've got → 'I got', What do you want? → 'What you want?' Such changes, initially regarded as examples of careless speech, can become so widespread that the whole grammatical system of a language is affected. Anglo-Saxon verbs, nouns and adjectives had sets of endings showing their grammatical roles; these have virtually all disappeared, and their functions are now expressed by auxiliaries or word order.

Reduction of complexity operates at a structural, as well as a phonetic level. Languages make many small distinctions which contribute little to communication, and speakers can easily feel unconsciously that it is too much trouble to get them all right. In modern English, irregular verb forms such as sank/sunk or sang/sung are being increasingly confused, resulting in utterances like *She sung very well in yesterday's concert. English modal verbs express numerous subtle shades of meaning which can become blurred. May have ... + past participle normally refers to a possibility that something really happened: for example (after an accident) They've taken her to hospital – she may have broken her leg. But people are now starting to use the structure to refer to an unrealised possibility, as in You were stupid to go skiing there – you may have broken your leg – a meaning that older speakers would only express by saying you might/could have broken your leg. Other current changes in small corners of grammar include the gradual replacement of inflected comparatives and superlatives (commoner used to be commoner, but more common is now more common), and a growth in plural marking on noun modifiers (antiques shop, drugs problem, arrivals lounge...).

Varieties of a language influence each other. Non-standard dialects converge on the standard, which has more social and cultural prestige and is therefore felt to be more correct, elegant or desirable. Similarly, one national variety can influence another. British English is changing in various small ways under the influence of its powerful American cousin, so that some of the few grammatical differences between the two varieties are disappearing. British speakers today, for example, often use Do you have...? for current possession, as in Do you have a match? For an earlier generation, Have you got a match? would have been the normal form. Similarly, the use of like to mean 'as if' (as in He looked like he'd seen a ghost), which has become widespread in informal standard American English, is now increasingly common in Britain. British-American differences in the use of must and have to are also becoming blurred.

Formal and informal varieties also influence each other. In 19th-century Britain and America the written language – the principal vehicle of legislation, social control and cultural transmission – had great prestige; informal spoken language was regarded as a poor relation, and the specific grammatical characteristics of speech were devalued. Today's oral media have done much to rehabilitate the grammar of speech, and this is influencing written norms, so that the gap between spoken and written varieties has been greatly reduced. For instance, the use of they/them/their with indefinite singular reference (as in If anybody phones, tell them I'm out), which has existed in informal speech for centuries, is becoming increasingly acceptable in more formal styles, as are structures with clause-final prepositions (something that teachers often worry about). Currently, the use of highly informal grammar in emails is further reducing the spoken/written divide, and may well influence the way people express themselves in more traditional letter-writing. (To say nothing of txt-msgs!)

As time goes by, old forms of expression get tired and worn out, and speakers of a language – especially younger speakers – seek to brighten things up. Creative innovation, therefore, is another powerful mechanism of language change. Present-day English seems to be developing a new future auxiliary. It is very common, especially in journalistic writing, to read that something is 'set to' happen: interest rates are set to rise, pub opening hours are set to change. Not long ago, this was a metaphor (referring to a runner in the 'set' position just before the starting pistol is fired), used only for people who were ready to do something. Now it is losing its original meaning and becoming grammaticalized as an auxiliary, used not only for people but also for things and processes. In another interesting change, standard English may be reintroducing the second person singular-plural distinction which it lost two or three hundred years ago. It is becoming increasingly common in casual speech for people to say you guys (to men or women indiscriminately) instead of just you, so perhaps this expression is slowly turning into a piece of grammar: a new pronoun.

What should we be doing about all this? Teachers don't need to worry too much – English as a whole is not changing fast, and most of the language will stay the same for some time to come. But it's important to keep an eye on what is happening, so as to be able to answer students' questions about new usage, and to be able to modify traditional explanations as this becomes necessary. From this point of view, the authors of reference books carry an important responsibility: they need to be continually alert to the small changes that are going on, in order not to find themselves giving misleading information that is no longer accurate. Writers such as myself are greatly helped in this endeavour by their correspondents. While working on the new edition of Practical English Usage I have been fortunate enough to receive a large number of useful questions and suggestions from English-watchers round the world drawing my attention to areas where descriptions of usage may need to be updated. My thanks to all of them.

By Michael Swan