Over here it breaks down account status into three things.

http://support.enmasse.com/tera/tera-rising-play-for-free#Do_I_have_to_do_anything?

I have standard since I did not purchase anything for tera. It seems standard really has a lot less compared to the other two. Is it really free 100% then?

If I purchase 1 month of elite will I get switched to standard after or founders?

Really not sure what your asking.

F2P - Basic Playing, Can still do everything in game.
Elite - Free 280 Speed Mount, A Thing that allows you to teleport to any big city instantly for free with no cool down, 3x boosts 1 hour per day, dungeons re-set faster and some other cosmetics. (Monthly payment of $15 per)
Founder - Can't be purchased from them directly, but still can be purchased from people. This is a one time purchase that gives you title's and other cosmetic items.

elite is good for quick leveling and farming gold at later level during quests

also boosting up your rep exp/point rewards to get nice accessories from rep vendors.

I play on a F2P account and you can honestly do everything the Elite players can. There's no special areas for Elite players and it really is %100 free.

I think Elites get some exclusive mounts and costumes but for me that's not really important.

As for what happens after your month of Elite runs out, I think you just go back to standard as Founders were people who bought the game before it was F2P.