Ok...
u don't need to look at SVA or some shit. The word, you, as in second person, is considered singular/plural as the case may be but then by default, in order to show the addressee some respect, the number associated therein is plural as a single special case. That's why it is always, "you are, you were, you have, you walk" just like "they are, they were, they have, they walk" and NOT like "(s)he is, (s)he was, (s)he has, (s)he walks"
This follows the second-person address in many other languages also.
I just feel this doubt is just not eligible to be raised in a gmat forum. Dig up your class II textbook of English.
Period.