you are a...才對
Be動詞包括is,am,are.它們與主語的搭配如下：
I(我) + am;
he(他),she（她）,it（它）及單數(shù)名詞（如the girl,Jim,this book等）+ is;
you,we,they及復(fù)數(shù)名詞（如these girls,Jim and Lily,those books）+ are
以上是規(guī)則,沒有原因,記起來照著運(yùn)用就可以了.
參照上面的規(guī)則,you are a...,可以這樣造句：
you are a nice man;
you are a student,I guess.等等
如要用.is a .,可以這樣搭配：
he is a boy;
she is a student;
it is a desk;
the girl is so beautiful
等等.