From UsingEnglish's glossary, two fairer definitions: A descriptive grammar looks at the way a language is actually used by its speakers and then attempts to analyse it and formulate rules about the structure. Descriptive grammar does not deal with what is good or bad language use; forms and structures that might not be used by speakers of Standard English would be regarded as valid and included. It is a grammar based on the way a language actually is and not how some think it should be.
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Mister MicawberI am very prescriptive. I try to tell you what I think is good English. On the other hand, I try to find out what literate natives really say before I recommend it.