fall off:To become less; decrease (变少；降低) 例如：Stock prices have fallen off.The number of staff meetings fell off after a few months.（股票价格下降了.工作人员的会议在几个月后减少了.） fall down:To fail to meet expectations；lag in performance（失败：未能达到预想；在成就上落后） 例如：That young boy fell down on the job.那小伙子工作未能达到预想目标.
