A prescriptive grammar is a set of rules about language based on how people think language should be used. In a prescriptive grammar there is right and wrong language. It can be compared with a descriptive grammar, which is a set of rules based on how language is actually used.
A prescriptive grammar would reject ‘He goes...', meaning ‘He said', as incorrect language.
In the classroom
At higher levels it is useful to raise learner awareness of differences between prescriptive grammars and use of language. This can be done in an inductive approach, with learners identifying examples of language that doesn't follow rules from authentic listening or written texts. They can then produce their own ‘mini' descriptive grammars.