Mental Grammar
A speaker’s unconscious knowledge of how a language works. A sentence is grammatical in a language if it follows the rules in the mental grammar of that language. A sentence is ungrammatical if it breaks t hose rules: that is, if it is something that people would not say.
Colorless green ideas sleep furiously.,* Green sleep colorless furiously ideas.
I have a friend who is always full of ideas, good ideas and bad ideas, fine ideas and crude ideas, old ideas and new ideas. Before putting his new ideas into practice, he usually sleeps over them to let them mature and ripen. However, when he is in a hurry, he sometimes puts his ideas into practice before they are quite ripe, in other words, while they are still green. Some of his green ideas are quite lively and colorful, but not always, some being quite plain and colorless. When he remembers that some of his colorless ideas are still too green to use, he will sleep over them, or let them sleep, as he puts it. But some of those ideas may be mutually conflicting and contradictory, and when they sleep together in the same night they get into furious fights and turn the sleep into a nightmare. Thus my friend often complains that his colorless green ideas sleep furiously.
Prescriptive Grammar
Someone’s ideas about what makes for correct or good writing.
Descriptive Grammar
A description of the rules people actually follow in normal language use.