1. The grammar of a language is the way its speakers put together units of meaning to form words and the way they put together words to form sentences. It contrasts with style, which is the way in which users of the language choose constructions from the grammatical repertoire and words from the lexical repertoire for their various communicative purposes. Grammar is a matter of fact. Style is a matter of opinion.
2. There is not just one English. There are many varieties and sub-varieties, and they vary according to geographical location and social class. These varieties have internally consistent grammars, and all thus have equal linguistic validity. Within any one variety, including Standard English, there are many styles, such as formal and informal, elegant and inelegant, effective and ineffective, friendly and distant, deferential and egalitarian, male and female.
3. Standard English, although a minority spoken dialect, is extremely important. It is the variety used in most published writing and it is readily understood by disparate geographical and social communities. There is widespread agreement on what constitutes it. The points which are hotly disputed are mostly questions of style such as the informal who set against the formal‘whom.
4. The terms ‘correct’ and ‘incorrect’ are unhelpful in discussing language, except in speaking of infants and foreign learners. Language is best judged on its effectiveness. To say that a particular usage is incorrect is inadequately descriptive and insufficiently damning.