from http://en.wikipedia.org/wiki/You
English and Dutch are similar in that both lost their old second person singular forms (those relating to the word "thou"), due to the use of the second person plural form as singular formal, with the plural ultimately replacing the singular totally as the informal forms came to be viewed as impolite. Ironically, this did not happen in French, the inventor of the formal plural; it has kept the system intact. Vous is still used as formal and plural, while tu is used for informal singular. Russian uses the French system also; vy (вы) is formal/plural and ty (ты) is informal singular. This probably resulted from the Russian aristocracy's use of French in Czarist Russia, and was likely strengthened by the T/V similarity in the French and Russian pronouns. This kind of system is also found in other languages, like Finnish and Swedish.
While English, Dutch, French and Russian use or have used the plural forms as the polite forms, other European languages use forms deriving from the third person. German, for example, uses the third person plural pronoun sie, capitalized Sie, as its formal pronoun (in other words, Sie literally means They). Danish and Norwegian languages similarly use De. Italian has separate forms for singular (Lei) and plural (Loro), which are derived from the Italian words for she and they respectively; a partial similarity to the German system (especially since the German word for she is also sie, but conjugates differently from Sie). However, sometimes the French system is also used in Italy, using the plural pronoun voi as singular.
Spanish and Portuguese use actual words which take third-person forms, and each has singular and plural forms. For Spanish, it is usted (pl. ustedes), and for Portuguese, você (pl. vocês). As in English, they seem to be supplanting the original second-person pronouns, which are now informal. The original Spanish second-person plural pronoun, vosotros, is now used only in Spain. Portuguese is farther along in losing them; the plural pronoun vós is gone totally in Brazil and used sporadically in Portugal, while the singular tu is dying out in Brazil, used sporadically in the southern region and certain rural parts of the country.
