When a pawn moves two squares on its first move and comes next to an opponent's pawn, it may be captured "en passant."
It must be captured on that move, you can not wait a turn and then capture "en passant."
If a pawn moves one square and comes next to an opponent's pawn, it may not be captured "en passant." It is only when the pawn moves two squares, on its first move that "en passant" may occur.
You do not have to capture.
