分析方法：语境与 字面
抓住 down off away 三者的区别
fall down

倒塌; 跌倒
Many buildings fell down during the earthquake.
在地震中许多楼房倒塌了.
失败
His plans have fallen down.
他的计划失败了.
fall off
跌落, 滚落下
When you are learning to ride a bicycle, you often fall off.
初学骑自行车时, 常会从车上掉下来.
突然降低; 向下斜
The land falls off here towards the river.
这儿的地势朝河的方向低了下去.
减少
The number of passengers by this line fell off slightly in January.
一月份这条航线的乘客数量略有减少.
变坏; 恶化
The cooking fell off remarkably.
烹饪质量大大降低.
逐渐消失, 死亡
Words fall off by disuse.
词不用就会成为废词.
不再支持; 叛离
His friends fell off one by one.
他的朋友一个一个地跟他疏远了.
变瘦; (健康、精力等)衰退
He took care to keep the cattle from falling off.
他精心照料着牛群, 不使它们瘦下来.
fall away
离开
All his old friends fell away from him.
他的老朋友全都离开了他.
消瘦
You'd scarcely believe anybody could fall away so rapidly.
你简直难以相信一个人会消瘦得这么快.