Yes, a very good try but not quite correct. "He joined the army two years ago" is good. It is simple and tidy, but does not indicate whether he is in the army now or not. "He has joined the army" sounds recent (like a few days or weeks ago), "He had joined the army two years ago" sounds less recent but also feels like he is not in the army now.. It is possible to say "He has been in the army since he joined two years ago" which is more complicated but shows that the time started 2 years ago and is still going. Also "since" should generally preceed "I/he/she/it/they/you + [verb]" or "the/an + [event]", it doesn't fit with "years ago" very well. Good luck with your study! ^_^

WHEN YOU USE "SINCE" YOU SHOULD SAY THE EXACT TIME OF BEGINNING.YOU CAN SAY : HE HAS JOINED THE ARMY SINCE 2007. HE HAS JOINED THE ARMY FOR TWO YEARS.

"he has joined the army two years ago''.