|"Or is it supposed to be 'Thou wilt not...?"|
Anyone can make a mistake, but to really foul things up, you need an expert.
In 1589, William Bullokar wrote the first book on English grammar, which was a start, but other experts spotted the defects right away. For one thing, Bullokar had written it in English, which was the sort of mistake you'd expect from an amateur, but his successors corrected this by writing their English grammars in Latin, which anybody could see was a much more sensible approach.
By the 1700's you couldn't throw a brick without hitting an expert in English grammar. Grammarians are like lawyers. If there's only one, nothing much happens - it takes two or more on opposite sides to really get the ball rolling, and once you have a sufficient quorum, they can brings things to a complete standstill.
The reason for the profusion of these experts was a crying need to "fix" the English language, not fix in the sense of "repair," but in the sense of pin down, keep from moving. English had gone from, "Hwæt! We Gardena in geardagum," to "Whan that Aprill, with his shoures soote the droghte of March hath perced to the roote," to "What light through yonder window breaks," in just a few hundred years. Anyone could see that if this precipitous rate of change were left unchecked, English was capable of mutating into anything; why, someday people might be writing nonsense like. "lmao" or "C U L8R" or maybe just smiley faces and stuff. This had to be prevented.
The only catch was, if you're rooting around for official rules for a language that has no official rules, where do you turn? The answer, other languages. For example, mathematics. In many languages, such a Spanish, you can pile on as many negatives as you please without changing the meaning one iota. No me cuenta algo, "they don't tell me anything," means just the same as, No me cuentan nada, "they don't tell me nothing," which means the same as No me cuentan nunca nada, "they don't never tell me nothing;" although admittedly the last one starts to sound a little petulant.
But in English, if you say, "They don't tell me nothing," some wise-ass is bound to point out, "Well, that means they do tell you a little something." The reason two negatives make a positive in English is because that's what they do in mathematics.
Mathematics is all well and good, but the go-to source for the early English grammarians was Latin. Latin is a dead language, which makes it a perfect model for a living one. The problem with something so sprightly and fluid as a spoken language is it won't hold still long enough to get a really good look at, unless you wait for it to be in its coffin.
For example, you're not supposed to end a sentence with a preposition. Maybe that rule makes sense to keep us from saying, "Where are you going to?" instead of just "Where are you going?" but what if you need to ask, "What's this book about?" Are you really supposed to say, "About what is this book?" Ending a sentence with a prepostion is perfectly grammatical in most Germanic languages, and English is a Germanic language, but you can't do it in Latin, so you're not supposed to do it in English either.
Latin is also where we got the peculiar rule to never, ever split an infinitive. The reason you're not supposed to split an infinitive in English is because in Latin, you can't. The infinitive "to go" is two words in English, but in Latin it's just vadere. You can't say, "to boldly go" in Latin unless it was something like va-boldly-dere, whatever the Latin word for boldly would be, which I do not intend to look up. But in English you can split the infinitive, only you may not.
May and can is another one.
These words were used pretty much interchangeably until the 1600's when some grammarian decided can, which comes from a root meaning, "having the knowledge to do something," referred to ability, and may, from a root meaning "having the power to do it," referred to permission. But it is not grammatically wrong to say, "Can I go to the bathroom?" when asking permission. Do you hear that, Mrs. Othmar, my third-grade teacher? There's nothing grammatically wrong with it.
Anyway, the grammarians sought to help us out and bring order to the mare's nest of English language, and to a large extent they have given us useful rules to follow. But keep in mind, English is not math, and it's not Latin, but its own unique thing, and if some petty snoot wants to sneer at you for perfectly sound English that doesn't happen to conform with some arbitrary injunction set down four hundred years ago by some self-appointed expert, use the retort Winston Churchill made when someone tried to correct his grammar, "This is the sort of thing up with which I will not put!"