Polite vs. familiar forms of "you"
It is true that the English we now speak has only one form of the second person, but it was not always so. The singular form "thou" with its other grammatical forms, and its equivalent verb forms (e.g., "thou art") began to disappear early in the 17th century and was just about completely gone by the end of the English Civil War, except for certain formal purposes, such as prayer and other liturgical use. (Still being used to this day a good deal for liturgical use in many Protestant churches.) You can track the process in the work of a single writer, John Donne. In his early work, he is using Thou consistently, as time goes on, less consistently, so that by his last years, ca. 1725 to 1730 (I think that was the year he died) he is using "you" pretty consistently, but not exclusively. The point(s) I'm trying to make here is 1) it is a fairly recent (as these things go) development; 2) the change involved simply dropping the singular form and using the plural form for both singular and plural, replacing the nominative "ye" with "you".
The French "vous" used for a polite form is the plural, but it didn't come to be the polite singular form the same way the English plural came to be used for singular. It comes from the old practice of Highly-placed Persons referring to themselves in the plural ("We are not amused") and their inferiors addressing them in the plural. So when you addresss a Frenchman in the plural, it's a compliment, inferring that he is a high-born or high-placed person. The German "Sie" is likewise a plural and comes to be a polite form for similar reasons, but also allied with the notion you get with the Italian polite form, "lei", which is 3rd person singular feminine and is a sort of contraction or substitution for "sua eccelenza - your excellency" or some such fawning honorific.
English, thus, when it dropped the old 2nd person singular, simply substituted the plural; it did not create a familiar or polite form, as the Continental languages, reflecting the social distinctions of the time, did.
