You are playing a game OTB and you play en passant.  Your opponent freaks, "that isn't legal he screams!" 

 What do you do?

You tell him to look it up on wikippedia or to ask a chess coach if it is a legal move. It is.

I was playing a game at a coffeehouse not long ago and my opponent got confused on en passant. I think he tried to play it when it wasn't legal. We just called over another player to settle it (don't let them call over someone else who doesn't know the rule!).

like any castle case, he must learn the rules, 'en passant' is the last thing players learned ;)  I heard a lot of dumbs screaming like that

What is "en passant"?

http://www.chess.com/article/view/chess-rules--basics

 

Scroll down to en passant.

True. I didn't learn about tell I read an old chess tutorial book.

If it's a tournament, just call over the tournament director. If you're playing a non-tournament game at a club or something, there's probably a book lying around that you can show him that explains the rule to him.

 

--Fromper 

yeah, that has happened to me before. I just told him to look it up.

I'd resign and say something about my opponent being an expert and too smart for me. Then I'd go find somebody else that would explain it to him. He'd feel about 2 centimeters tall.