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So, I am trying to establish the difference between these words and when you would use one over the other. Here are a few things that I have established so far:
only 以前 can be used with a period of time (e.g. 一个星期以前)
they can all be used colloquially, but 曾经 sounds more formal than 以前
And that is about as far as I have got...
I suppose my main question is when to use 从前 and when to use 曾经 Any insight is greatly appreciated!
以前 does not have to be used like in 一个星期以前 - you could say “我以前有吃过” meaning "I've eaten it before". Using 以前 usually tells you that it wasn't too recent, maybe referring to something at least a month or two ago.
I would translate 曾经 as "recently" - e.g. “我曾经吃过” meaning "I've eaten it recently", and that would refer to something within a couple weeks. However, I wouldn't use it if I were talking about something that happened today or yesterday.
As for 从前, this is very rarely used colloquially in speech - I always translate it as "once upon a time". You might find it at the start of a story (e.g. “从前，有一只灰色的小猫” meaning "once upon a time, there was a grey kitten") but is not used a lot otherwise.
I would like to modify mbjb's answer about 曾经.
曾经 should be translated as once instead. “我曾经吃过” should mean "I have once eaten" ("I have eaten before" is correct as well, but to distinguish the differences, once is more accurate)
Although similar, 曾经 and 以前 are not interchangeable. Take a look at the following sentence:
"我以前经常来这操场玩，而且曾经在这里摔倒。" I often come to this playground to play in the past, I also fell down here once.
You cannot switch the 曾经 to 以前 here, it cannot give the meaning of "once".
以前 and 曾经 can actually be used together as well!
"我以前曾经在这里摔倒。" I have fell down here once before
I am sorry that I don't have enough reputation so I cannot comment.
