fall down 是 倒塌; 跌倒 跟 失败
例句：
Many buildings fell down during the earthquake.
在地震中许多楼房倒塌了.
Babies often fall down when they learn to walk.婴儿在学走路时经常摔倒.
His plans have fallen down.
他的计划失败了.
come down 是 下来 倒下 下落 ==
例句：We go up and come down by lift.我们乘电梯上下