nothing but =only是固定短语——无所谓指人还是物啊!
例如：There is nothing but bread left.=There is only bread left
nobody but不是固定短语应该是除了...以外没有任何人.
Nobody but only John was studying in the classroom.
注意有一个短语是 anything but=not at all是高中的一个难点.
例如：It is anything but a car.