Distinguishing lie lay lain and lay laid laid in common usage?

No one ever gets these right, at least the use of "lay" and "lie." Doesn't usage finally overtake grammatical rule and establish a new "correct?" Two others are "its" and "it's" and using "you and I" in the objective case instead of "you and me". I believe that replacing "who" with "that" has already become standard usage. Grammatical errors, along with atrocious spelling, glare out at me, but I feel I am becoming archaic, and already I have become boring whenever I speak up. Yet I want to stand in for those venerable old English teachers who dedicated themselves to my proper use of English. Linguists (and before them, philologists) doent such shifts that have occurred over historically scaled time -- English and its progenitors have been losing case and tense distinctions over hundreds of years, for example, and there have been pronunciation shifts. Who decides and how is it decided when a new usage becomes correct?