1.My birthday is [on] December 11th
具体日期用on（你打错了吧,December拼写错误而且开头没大写）
2.Today is my grandpa's [sixtieth] birthday
应该表示成第六十个生日,用序数词sixtieth
3.What [are] you doing now?
对于第二人称you,使用的be动词形式是are
4.I'm going to [draw] a picture for christmas day
be going to后面加动词原形表示将要做某事