What is good English?

Every teacher of English, particularly if he is teaching foreign students, must have been asked the question "What is the correct pronunciation of ----?" or "Is it good grammar to write ----?" and on giving his answer must have been confronted with the reply "But I have heard many Englishmen pronounce it differently" or "But this very eminent novelist breaks that rule; who is finally to decide which is right?" The answer, of course, is "No one". There is no Academy or other body in England to determine the correct form. The chief criterion of correctness is established usage. Correctness in spoken English is conformity to the speech usages of the majority of educated people; correctness in written English is conformity to the usages of the best modern writers. The rules of grammar are like the laws of Nature. The laws were not made for Nature to obey, but are simply a few facts which wise men have observed as to the way Nature acts. So the grammarian merely examines the language of the best speakers and writers, and deduces rules from their use of it.

Custom is the basis of these rules, and custom is always changing. Pronunciation changes from generation to generation, words decay and become obsolete, and newcomers thrust their way in; words acquire new meanings, sentences are constructed on different lines, and even the syntax of the language undergoes modifications.

It often happens that different forms are in use at the same time, differences due to regional or class dialect, though owing to the modern ease of communication and consequent intermingling of people, the spread of popular education, and the hearing of the "Standard English" of the B.B.C., dialect differences tend to disappear.

Again, there is a difference between the language used in writing and that used in speaking. In written composition the words will naturally be chosen with more care and used with greater precision than is possible in rapid familiar conversation, and the sentences will tend to be longer, more elaborately constructed and more conservative in their avoidance of "colloquialisms" and slang. To write as we talk would be slipshod; to talk as we write would sound pedantic and unnatural.

It is the business of the grammarian to observe and record these changes and differences and to decide as far as he can what is the form of language used by the majority of educated speakers and writers; and their usage is his only authority for saying what is "good" and what is "bad" grammar.

Quoted from A Concise English Grammar for Foreign Students by C.E. Eckersley, M.A. (Longman)