When you but a game on steam do you have to pay monthly or once you buy it you get to keep it and play it forever

When you buy a game on Steam, it's yours to keep and play indefinitely. Steam is free. The only thing you have to pay for is the first payment to get the game. Unless next to the pricing of the game it says "reccurring subscription fees apply"

IE: Team Fortress 2 is $20.00. That is the only payment you will have to make for it.