Jump to a discipline Disciplines Art Biology Business Chemistry Communication Comm. Disorders Dance Education English For. Languages Geology History Humanities Math Music Philosophy Pol. The grammar of any language is the set of underlying rules that make possible meaningful utterances in that language. You know how to combine nouns, verbs, and modifiers to create sentences, even if you couldn't define the terms noun, verb, and modifier.
Usage is a much more slippery concept than grammar.
It has to do with habitual or customary practices in spoken or written language. The concept is important for understanding how language works, because members of a language community, who by definition all share more or less the same grammar, will nevertheless differ when it comes to certain specific modes of expression. Yet, at least in written English, ain't is widely frowned upon as unacceptable usage. The answer tautological as it may sound is that it is not accepted. By those in a position to make some usages acceptable and others unacceptable that is, teachers, editors, compilers of dictionaries and, of course, authors of usage manuals.