en passant happens when a pawn uses its first-move option to move two squares forward instead of one. if it passes over a square guarded by an enemy pawn, that pawn can take the first pawn "in passing" "as if" the first pawn had moved forward only one square. the taking pawn moves into the empty square over which the first pawn moved. the first pawn is removed from the board. to capture "en passant" can only be done on the move right after the double-square pawn advance. otherwise the chance is lost.