What is the en passant pawn capture? To understand it, we need to think of the normal way that a pawn moves forward (when it is NOT capturing): one space forward, straight ahead, towards the side of the board where the enemy’s pieces were set up at the start of the game. The exception to that one-space-forward movement of pawns is when a pawn moves from its original square, for then it can more either one space or two spaces forward. That is the key to understanding how such a pawn can be captured en passant.
This kind of pawn capture must be under the following conditions:
- A pawn captures an enemy pawn.
- The pawn being captured (and removed from the board) had just moved two spaces forward from its original square, and the capture must be done immediately after that two-space move. That two-space move itself was NOT capturing anything.
- The pawn doing the capturing moves just as if the captured pawn had moved only one space forward and was then captured (in the normal way) by the capturing pawn.
In other words, the en passant capture applies only when one pawn captures another pawn, and it prevents a pawn from moving two spaces forward to avoiding being captured by an enemy pawn. If the capture is delayed, its right is forfeited. If it is to happen at all, the en passant capture must be done immediately after a two-space-forward pawn move.