I "played" D&D once with some people who play quite a bit, and was confused. Maybe I was expecting more clear rules of how to "win"? It seemed like it was more of a "choose you're own adventure" story rather than a game since the dungeon master is able to change the rules as you go and pretty much decide whether or not you are "successful" with whatever goals are established at the beginning. Is that accurate? In the…
It's not exactly accurate that the DM can change the rules as you go. The DM will sometimes say you can't do x or y, but a good DM doesn't do that capriciously. When I DM, I sometimes tell players flat-out "you can't do that", but it's because they've tried to do something which isn't possible in the fictional world (like if someone says they want to jump so high they land on the moon or whatever). Most of the time, when a player says they want to do something, they should either succeed outright (because it's not actually challenging for the character to do), or the rules of the game will give a clear way to determine success/failure. It sounds like you maybe didn't have a very good DM.