What is the essential relationship between rules of grammar and a living
It is possible to have a view in which a grammar has some description, and a bit of prescription. If someone speaking English used only "sentences" without verbs, then a bit of prescription might be in order, and the response that "grammar is descriptive not prescriptive" would or should fall on very deaf ears. For the language at the moment does have verbs. And the same is true if someone confuses "infer" and "imply", for example. On the other hand in American English "insure" has replaced "ensure", so "insure" in US parlance now has two meanings, and one of them ("to make sure that") may be in the process of being forgotten. I find this hard to take, myself, but a descriptivist has a case here too. It is obvious that most living languages do change over the years by shifts like this one. But the basic structure of the sentence is fairly stable over a long time. I do not think that English could suffer the loss of all its verbs and remain in any reasonable sense the same language. About changes over time: Shakespeare was the first to use "ruin" as a noun, for a fallen-down building. That took some imagination and some time, from e.g. "the ruin of my house"; was the usage wrong when it came to mean "my castle falling down"? Certainly not for us; we use the word "ruin" of the stones and other materials left over from a building that has collapsed all the time, and perhaps also of the process, the ruining. 'The ruin of Widdershins Castle took place over a whole century.'