Pot-Limit Betting
If no one has entered the pot yet, the most a player can bet is the size of the pot. Let's say we're heads up on the river and there is 1,000 in the middle. You check to me on the button. The most I can bet here is 1,000. Simple enough.
What sometimes gets tricky is figuring out in pot-limit games how much a player can raise after a bet has been made. Let's say I do bet the pot (1,000) in that previous example, then you decide to check-raise the pot. How much can you bet?
For you to bet pot, you'd be putting enough chips in the middle to call my raise (1,000) plus the equivalent of the new pot size after your call -- in this case 3,000. So for you to check-raise the pot, you'd be betting a total of 4,000. (And I thought this was a friendly game.)
Remember that preflop, the blinds constitute the initial betting, so once again, for someone to open-raise the pot, that player would be betting the equivalent of the big blind plus the new pot size. For instance, here in the PLO half of Level 1, the blinds are 25/50. So to open with a pot-size raise, a player would be betting 175 (50 + 125).
"String bets" are not allowed at the WSOP -- in other words, players cannot make multiple moves when betting, like one often sees in the movies (e.g., "I call your 50, and raise you another 125.") However, it is sometimes helpful to think of the pot-sized bet in two parts when calculating it (a call plus a raise).