Bist Du allein?

Another of those awkward "granddad" questions - why when all the other European languages have separate words for you (singular) and you (plural) do we not?  Well the answer is that we used to - the singular "you" used to be thou, but now, apart from some local dialects, that's really only used to address God.  You, which has always been the plural is now used for both, which is why we say "you are" (plural) even when addressing just one person.