So I was thinking about how we lost the informal second-person pronoun the other day, thou, and about how du/you and Sie/thee are "false friends" (words in two languages that look or sound similar but have different meanings). Then realized "thee" is the accusative form, so the pair would be Sie/thou which wouldn't be confused. (Yes, I am an all around geek.) So I went to the Internet to see what I could find.
What I found on the Online Etymology Dictionary was interesting. We lost the informal form of address because it came to indicate a difference in status between the speakers (a king would address a servant as "thou" but the servant would use "you" to address a king) and not as how one addressed ones equals. As we moved away from the feudal system and everyone wanted to be treated as equals, at least socially, "thou" faded from use. We dropped the informal you as society became, in some ways, less formal!
Now in German speaking countries the formal you (Sie) is falling into disuse as society becomes more informal. It is becoming less common to use formal address when speaking with an equal whereas traditionally the formal form of address was used with almost anyone who wasn't a friend or relative. An acquittance will ask the other "Sollen wir dutzen?" (Should we use "du"?) when he feels they are now friends, at least according to my German teachers. It's similar to saying "please call me Ann" after having always referred to one another as Mrs. Jones and Ms. Smith. Just as it is always safest to be a bit formal in and use last names in English, it is still safest to use the "Sie" when speaking to an acquaintance but often they will use "du" without batting an eye, particularly in an informal setting.
Another cool thing is that similar to German*, the formal 2nd person singular pronoun is the same as the 2nd person plural pronoun. This is why "you" can be very confusing and why Southerners say "y'all" and New Jerseyites say "youse guys" when referring to groups of people (and, yes, sometimes individuals). There is no good way to separate the two, we even conjugate many verbs the same. (Actually we conjugate many verbs the same regardless of the subject.)
And now I have a question for you (all 2 of you regular readers) -Tell me about the pronouns in the language(s) you studied in school. Does it have a formal and informal 2nd person singular pronoun? Is the formal also the plural 2nd person? Are they both still in use? How are they commonly used? Do you even remember the pronouns in the language you studied?
Did I use "cool" enough in this post?
*In German "Sie" is the formal 2nd person singular and "sie" is also the 3rd person feminine (her) and 3rd person plural (they). With "she" verbs are conjugated differently than with "they" and "you (pl)", which are conjugated the same.