为何不能如下scala代码会报错？为何不能省略override关键字？

abstract class Teacher{
  def teach:Unit
}

trait ChinsesTeacher extends Teacher{
   def teach: Unit = println("teach Chinese")
}

trait EnglishTeacher extends Teacher{
    def teach: Unit = println("teach English")
}

def main(args: Array[String]): Unit = {

      val teacher=new Teacher with ChinsesTeacher with EnglishTeacher

      teacher.teach

}