Zhiqian, yiqian and congqian

之前／以前／从前

以前 can be translated as Before, 从前 is usualy used when telling a story, it can be translated as „Once upon a time“. 以前 can replace 从前 almost in every case, but 从前 can be used only in a specific situations.

从前 can be used only if it stands alone and can not be acompanied by another time expresion.

There used to be a castle before.

对 － 从前在这里有一座城堡。

对 － 以前在这里有一座城堡。

There was a castle two years ago.

对 － 两年以前这里有一座城堡。

错 － 两年从前这里有一座城堡。

以前／之前

以前／之前 can be alo used as a definition of a time in a future as well as in a present. 从前 does not have this usage. For Example:

对 － 我想明天两点以前走。

错 － 我想明天两点之前走。

错 － 我想明天两点从前走。

The difference between 之前／以前 is very easy. 之前 expresses longer time.

实用对外汉语教学语法(pp. 54) 对外汉语教学语法释疑201例 (pp. 3, 5, 7)