可以这样改：Helli,everyone!I am on duty today.Today is Monday,December 21 th 2009.We are all here.I have a good friend,her name is Wang Min.We call her Lu Dan,because she has a round face.Her hair is short and black,she has a big nose,but her eyes are small.Her Chinese is good and my English is good,so she helps me with my Chinese and I help her study English.We help each other.Her birthday is on Christmas,I wish her a happy birthday.我有一个好朋友,她的名字是王敏.我们叫她卤蛋,是因为她有一张圆圆的脸.她的头发又短又黑.她的鼻子很大,但是她的眼睛却很小.她的语文好,而我是英语,所以她帮我学习语文,而我帮助她学习英语,我们互相帮助.她的生日就在圣诞节,我祝她生日快乐.Christmas是具体某一天,前面的介词应该用on 不是感觉,你是学生吧?就中学生而言,记住具体某一天前的介词用on,除非是固定的词组