1. There is not one English, but there are many varieties and sub-varieties of English. They vary according to geographical location and social class.
2. Within one kind of English, there are many styles (formal/informal, elegant/inelegant, friendly/ distant, deferential/egalitarian, male/female).
3. 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. Grammar is sometimes used to mean a set of rules about Standard English, but that use can be misleading and is best avoided.
4. Standard English, although a minority dialect, is extremely important. It is the variety most readily understood by disparate geographical and social communities. There is not, however, total agreement on what constitutes it.
5. A large part of the skill in written and spoken language lies in knowing what style to use for a particular purpose. ‘The boy done good’ is entirely appropriate at a football match in a way that ‘That player seems to have played remarkably well’ would not be.
6. The terms ‘correct’ and ‘incorrect’ are not helpful in discussing language. Language is best judged on its effectiveness, on whether it does the job that its users intend. Standard English, well handled, will often achieve that purpose. But to say that a particular usage is incorrect is inadequately descriptive and insufficiently damning.