Many western languages have kept their formal and informal registers -- French with Vous and tu, Spanish with Usted and tu, German with Sie and du. English has lost this usage almost entirely.
In the 17th century, “you” was the plural and the formal pronoun. One can still get a sense of the old plural = formal link in the way that the Queen of England, for instance, refers to herself (very formally) in the first-person plural: “We are not amused!”.
“Thee”, “thy” / “thine”, and “thou” (in the objective, genitive and nominative cases, respectively), were singular and informal. As one still does in other languages, formal pronouns were used with social superiors and casual acquaintances, and informal pronouns with servants, animals, and close friends and family.
Quakers, to emphasise the equality and amity of all mankind, insisted upon addressing everyone with the informal pronouns; many were quick to take offense.
Ironically, the disappearance of informal pronouns from ordinary speech means that today most modern English speakers encounter them only in formal settings, such as religious or poetic discourse. This set of circumstances leads most people to associate “thee” and “thou” solely with contexts of solemnity or formality, precisely the opposite of their original meaning!
Regular usage of “thee” and “thou” persists in only a few, relatively isolated linguistic contexts: the dialects of Yorkshire, Northern Scotland, and a few regions of Appalachia.
The lack of a singular / plural second-person pronoun distinction in modern English has led most speakers to adopt, at least colloquially, various dialectical plural pronoun constructions: you lot, you guys, y'all and so forth.
