"He participated in the competition a month ago."
Exactly. I put that in and I got it wrong... I'm pretty livid atm lol. I'm native Chinese and, it aint too far of a stretch to say that it's kinda outrageous that I'm being denied the rights of my own language
他一个月前参加比赛了 = he has already competed (so he doesn't need to this time) 他一个月前参加了比赛 = he participated in a competition a month ago (the competition could still be going on)
This is a good simple way to explain the craziness of "le." You're a better teacher than Duolingo right now, julykix -- just gave you a Lingot :-)
Since this is a past action, the 了 can go directly after the verb, or at the end of the sentence. Both should be accepted.
i think duolingo is wrong or failed to accept other natural answers. So you might not have learned anything new here.
他一个月前参加比赛了。If you're going to force “了” at the end the translation should be. He HAD participated in the competition a month ago. Seeing as how you mark every other thing wrong for inconsequential changes.