Dear Heloise: King Alfred the Great would never have said “you” to one person, since this was the plural form, and “thou” was the singular form. (This can be compared to the German “du” and the Spanish “tu.”) After the French-speaking Normans in 1066 forced a two-tier society on England, they demanded that they should be addressed with the plural pronoun, while they addressed the conquered natives with the singular (which explains why the plural “are” is still used with “you”). Examples of changes could be multiplied.