Boy have they atrophied. Even as a German speaker in whose first language these words have current equivalents I'm not 100% certain when to use thou, thee, thy, thine etc. that still were part of the language at Shakespeare's time and have since been simplified into you/your/yours etc. But it's true, English takes this stuff in stride, with modernisms e.g. "sick" meaning something good gradually being incorporated into the mainstream, rather than fought against by language purists.

thou/you is formal and informal and the distinction largely depending on your relationship with the person and respective social ranks. There were times when one person (social superior) would use thou (informal) while the other (social inferior) was expected to use you (formal). So yeah, no hard and fast grammar rule on when to do it but would depend entirely on the culture and the speaker and listeners social position inside of it.

No, initially thou was simply the singular, with you as the plural second person pronoun. You'd address one person as thou, a group of people as you (like some speakers use you vs y'all today). Thou, my friend vs You, my friends.

Then, under French influence probably, the plural, you, started being used as a polite form as well (in French, like most romance languages, formal/polite language uses the plural form of pronouns and verbs when addressing a single person). Thou, my friend VS You, sir; similar to "toi, mon ami" vs "vous, monsieur".

Then, this polite form using singular you became so widely used that thou was almost entirely dropped, especially since English also had little distinction between singular and plural in verbs in general. You, my friend, you, my friends.

Then, as thou became more foreign to regular speakers, it briefly started being used as a polite form, essentially reversing the original meanings. You, my friend VS Thou, sir.

This didn't last very long, so finally we ended up with the current state, where there is no polite form and you is the only second person pronoun. Except of course some speakers have started using y'all for a plural form, but that doesn't seem to be gaining any popularity outside a few areas.

There are purists that complain. Importantly, we don’t let them edit the dictionaries.

English is a barbarian language with French nouns, as a result of the Norman conquest of England.

Amusingly, using the French words is a signal to being upper class. Such as "purchase" (pourchacier) instead of "buy" (byan).

Or stuff like "cow" (from Old English) vs "beef" (from Old French). Which kinda makes sense when you consider who grew meet vs who ate it.

It's a pretty common thing worldwide, though. French played a similar role as upper class marker in many other countries that were influenced by it when France was at the peak of its global dominance. For Slavic languages, German also played this role at one point, and IIRC there is something similar historically with Chinese in areas in its cultural dominance.

All non-Greek languages are barbarian, since apparently it sounds like we are saying barbarbar to the ancient Greeks.

Imagine how those Greeks resented scribes trying to separate words and phrases with spaces on precious vellum!

Those crazy Masoretic Jews trying to pollute sacred texts with vowels... You're just supposed to know them!

Punctuation was probably introduced by leaky quills dripping until someone put a positive spin on it.

What twist of fate gave us ampersands? Lets keep Ye Olde English pure! https://en.wikipedia.org/wiki/Thorn_(letter)#Modern_English

Cutlery (fr), silverware (de)

I was taught that this is because the Normans pushed the Germanics out and up north. French dominated the royal court.