Translation:He participated in the competition a month ago.
One month ago, he participated in a competition. HELLO??? I understand that you cannot put every translation, but seriously, if you're an English speaker, you gotta have some complexity.
I don't know how they made Chinese work. I am fluent and they don't let me out with Chinese or English.
It is nicer/simpler to say "he competed" than "he participated in the competition." The only reason you would say "participated in the competition" is if you're emphasizing THIS competition for some reason. Like if you were talking about a particular competition and then wanted to add that this person took part in it.
"He participated in the competition last month" should definitely be accepted. Please bear in mind that we can discuss solutions here, but please press the "Report" button and mark it "My answer should be accepted".
ENTERED the game...??? ie came into the business??? Did a non native speaker overhear this sentence out of context?