Grammar is a Greek word "grammatike tekhne" which means "art of letters". In Middle English it came to mean "learning in general, knowledge. Its modern meaning as the "rules of language" is a post-classical development which was limited to Latin until 16c. 

Grammar is nothing but a game of observation. Play it, it is fun. Learn to look and listen carefully. Get involved to find out  more. Since grammars are written retrospectively through observation there is no complete grammar of any language. There is always something still hidden needs to be observed by somebody like you. Add your observation to what has been already observed. Correct previous observations if they were wrong. You are  a detective or the policeman/-woman who wants to find out the truth. You can see grammar from different prospectives: retrospectively, introspectively and prospectively.