Talking about things in the past - 曾经， 从前，以前
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 &quot;I have once eaten&quot; (&quot;I have eaten before&quot; 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:
&quot;我以前经常来这操场玩，而且曾经在这里摔倒。&quot;
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 &quot;once&quot;.
以前 and 曾经 can actually be used together as well!
&quot;我以前曾经在这里摔倒。&quot;
I have fell down here once before
I am sorry that I don't have enough reputation so I cannot comment.