you are a...才对
Be动词包括is,am,are.它们与主语的搭配如下：
I(我) + am;
he(他),she（她）,it（它）及单数名词（如the girl,Jim,this book等）+ is;
you,we,they及复数名词（如these girls,Jim and Lily,those books）+ are
以上是规则,没有原因,记起来照着运用就可以了.
参照上面的规则,you are a...,可以这样造句：
you are a nice man;
you are a student,I guess.等等
如要用.is a .,可以这样搭配：
he is a boy;
she is a student;
it is a desk;
the girl is so beautiful
等等.