Can I just state my agreement with Darwyn etc. If you don't understand a rule, there is a simple choice: You ask, or you guess. If I was in a game with somebody who did not know the rules, I would rather that they asked, and so the game was won on the basis of skill, rather than they guess, so the game was perhaps won on the basis of another person's simple mistake, irrelevant to the actual skills of the game.
To look at your chess example, do you want to win because somebody didn't know the en passant rule? Worse, in a tournament, to see an opponent win a game, and so beat you, because another didn't know the en passant rule? I would hope not.