En passant is a special pawn capture: a pawn that has just advanced two squares can be taken by an enemy pawn beside it, as if it had moved only one.
When a pawn makes its two-square opening jump and lands directly beside an enemy pawn, that enemy pawn may capture it ‘in passing’ — moving diagonally onto the square the pawn skipped over, and removing it.
The catch: you can only do it immediately, on the very next move. Pass up the chance and the right to capture en passant is gone for good.
The rule exists so a pawn can’t use its two-square jump to sneak safely past an enemy pawn that would otherwise have controlled the square it passed.
Only immediately after the enemy pawn makes its two-square advance, and only with a pawn of yours sitting directly beside it on the 5th rank (for White) or 4th rank (for Black).
No — it’s optional, like any capture. But it’s often the right move, and sometimes the only good one.
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