descend 通常指沿斜线或斜坡下降。
drop 指物体从一定高度落下。
fall 与drop同义，指突然或猛烈地降落，但fall也可指任何下落，同高度或形式无关。
sink 指在空气或水中垂直下降、下沉。
参考例句
1. The elevator descended slowly to the ground floor. 电梯缓慢地下降到底楼。
2. So shall he descend to your roots and shake them in their clinging to the earth. 他也要降到你的根下摇动你的根柢的一切关节使之归土。
3. They should also have a clear idea of how best to descend the towering inferno should such an emergency arise. 如果发生这样的紧急情况，他们也应该清楚地知道逃出这一高耸的地狱的最佳之道。
英英解释
v.
1. move downward and lower, but not necessarily all the way
2. come from; be connected by a relationship of blood, for example
3. do something that one considers to be below one's dignity
4. come as if by falling
