The other two are more successful than I am. Their books are published by Longman. Grammar describes the structural rules of a particular dialect of a particular language at a particular time at the sentence level. The central flaw in this definition is the assumption that we could actually capture a language in a grammar. Language is dynamic and constantly changing.
Every language is made up of several dialects, and at least one idiolect per speaker.
Thus, a description of the sentence structures of a language, even if it is correct for a particular moment for a particular speaker in a particular context, is outdated before it can be committed to paper. Prescriptive grammar prescribes what is right and wrong in the sentence structure of a language user. The assumption is that there is only one right response to a question about language usage. Thus, many grammatical rules which linguists, clergy, and other language authorities extracted from Latin were applied to English. And vice versa.