Can anyone date the invention of the current grammatical rules we have today in the English language?

Answer 1

This question has no direct answer. Grammatical rules are not invented for languages (with the exceptions of conlangs like Elvish, Klingon, etc.). Grammatical rules evolve. They are borrowed from other languages with familial ties, or are introduced by speakers of other languages and adopted into normal speech. Grammar is evolving all the time, and English has certainly not reached its final form, nor will it ever!