Page 323 - Practical English Usage 3ed - Michael Swan, Oxford

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divided usage
Speakers of a standard language often differ about small points of usage. Where two different forms are common, people who use one form may claim that theirs is the only 'correct' usage, and that people who use the other form are making mistakes. Some examples from modem English:
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In fact, all of the so-called 'mistakes' listed above have been normal in standard English for centuries, and are not wrong at all (though some of them are more informal than the so-called 'only correct forms', and would be out of place in a formal style). For details, see 429 (1 and me), 155 (different), 320 (less), 528 (their) and 467 (presently).
prescriptive and descriptive rules
If people say that less people or different to is wrong, they are following a prescriptive rule. Prescriptive rules are made by people who believe that they can improve a language, or protect it against change. A lot of prescriptive rules were made by eighteenth- and nineteenth-century British grammarians, often because they thought that English grammar should imitate Latin, which was considered a superior language. A typical example is the rule that 'split infinitives' like to boldly go, where an adverb is put between to and the verb, are wrong (a Latin infinitive is a single word, so cannot be split). Many people still believe this, and try to avoid split infinitives, although the rule is unrealistic (see 280.7). A similar rule said that sentences should not end in prepositions (as in What are you waiting for? or 1 don't like being shouted at.). In fact, it is quite normal for English sentences to end in prepositions (see 452). Most prescriptive rules give misleading information, and have little effect on the development of a language.
Descriptive rules simply say what happens in one form of a language (for example standard written British English, standard spoken American English, Yorkshire English, Dublin English or Singapore English), and not what some people feel ought to happen. The rules in this book are descriptive of standard British English.
When do mistakes become correct?
When somebody misuses a word or expression, this may influence other people to make the same mistake. Sometimes a mistake becomes so widespread that it becomes part of the language (this is one way in which
languages develop), and we can no longer realistically call it a 'mistake'. The expression oblivious of, for example, originally meant 'forgetful of, but came to be used to mean 'unconscious of. A hundred years ago this was still a mistake; now it is the normal use. The same thing is happening today with the expression a concerted effort. This means 'an effort by people working together', but some people now use it mistakenly to mean 'a strong effort'. ~
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so-called 'only correct form'
John and 1 went to the cinema. They're different from us.
fewer people
Somebody's dropped his or her
keys.
I'm unemployed at present.
so-called 'mistake'
John and me went to the cinema. They're different to us.
less people
Somebody's dropped their keys.
I'm unemployed presently.
kinds of English (2): correctness 309
page 291