- 从前= once upon a time (常用语故事中，书面语，也称为：曾几何时) - 以前= before, in the past（口语）

从前 is usually used in telling a story. In most occasions, if not all, it can be replaced by 以前。 以前 can be used to ①indicate time before now or a certain time.　② mean 从前 or 以往。 grammatically 从前 can be merely adverb, while 以前can be adverb or preposition（？i'm not sure, in fact we never know the grammar）, which means it is correct to say "三天以前" rather than "三天从前"...

从前，大部分都用在讲一个故事的开头，例如：从前有座山，山里有个庙... 以前，一般是为了和现在进行对比，经常是现在的状态和以前的状态是是不同的。例如：以前，我们经常去河里游泳。（但现在的情况可能是，河水已经被污染了，没有人会去河里游泳了）。

以前 you can use 1) indicating a definite time point, e.g. 兩個星期以前 2) you can use it to say that something is going to happen before a certain time point in future too. 從前 doesn't have the above options. Wait till mother tongue speakers explain better.