Originally posted by swinny
can someone explain in terms an idiot could understand
OK I'll take some time over it and see if it's comprehensible:
A few hundred years ago the rules of chess only allowed the pawns to move one square forward on their first move.
So in those days if you had a position like this with white to move:
from here if white moved the pawn on a2 - that's the one in front of the rook on the far left - it could only go one square forward to a3 - so it could then be captured by the black pawn on b4.
When the rules were changed to allow pawns to move either one or two squares forward on the first move (an idea to get the game going faster) some people complained that it wouldn't be right if you could jump past an opponents advanced pawn and avoid capture. So they made up the rule: En Passant - which is French for: in passing
So in the above example, with todays rules, if white moves the pawn to a4 then black can still capture - as if the pawn had only moved to a3 - the captured white pawn is removed and the black pawn then occupies the a3 square - again - just as if it had captured the pawn on a3
This is only allowed if it is done straight away...i.e. white moves the pawn to a4 and on blacks next turn it is allowed to capture "en passant". If black doesn't do it immediately it's not allowed later.
So when black has a pawn on the fourth rank and white moves a pawn on an adjacent file two squares forward - then - for blacks next move only - black may capture this pawn as if it had only advanced to the third rank. The same applies with colours reversed - when white has a pawn advanced to the 5th rank etc..
Players new to chess often forget or just don't know this complicated rule - so it can be a surprise or even the cause of an argument.