在表示价格或温度等下降时fall和drop是通用的。
The temperature dropped /fell to zero the next morning.
第二天早晨温度降到零度。
在其他方面区别如下：
1.fail只能是不及物动词；drop既可以是及物动词又可以是不及物动词：
She slipped on the ice and fell.
她在冰上滑了一跤。
Their profits have fallen by 30 per cent.
他们的利润减少了30%。
The temperature fell sharply in the night.
夜间温度陡降。
Output will fall by 6%.
产量将减少 6％。
It is important not to let production levels fall.
重要的是别让产量滑落。
I’m not going to fall, am I?
我不会摔跤吧，对吗？
In the short run, unemployment may fall.
从短期来看，失业率可能降低。
Share prices fell early on but rose again late in the day.
那天股票价格起先跌了，临近收盘时又涨了。
The climber slipped and dropped to his death.
攀登者一失足掉下去摔死了。
Temperatures can drop to freezing at night.
夜间温度可能会降到零度以下。
The temperature has dropped considerably.
温度已大大降低。
In November the temperatures drop and the days shorten.
十一月气温下降，白天变短。
At last the wind dropped.
风势终于减弱了。
Be careful not to drop that plate.
小心别把盘子摔了。
He dropped his pants.
他松开腰带，裤子掉下去。
A couple of boozy revellers dropped their trousers.
两名醉酒的狂欢者脱掉了他们的裤子。
Drop everything and come here immediately.
放下所有的事情立即到这里来。
Then the Class 2 runner dropped his stick on the ground.
然后，二班接力赛运动员的接力棒掉在地上。
2.当表示动物的词作主语时，不及物动词drop表示有意志的行为；而fall 只能表示非意志的意外动作：
A burglar dropped from a window to the ground.
一个入室盗窃者从窗口跳到了地上。
She slipped and fell(down).她滑倒了。
She fell asleep during the sermon.
在布道过程中，她睡着了。
I pinched myself to stop myself from falling asleep.
我掐自己以防自己睡着。
I went to bed early that evening, but dropped asleep late.
那天晚上我早早上床了，但是很晚才入睡。