Sentence:  [I]f the opponent chooses to make this capture, he must do it "immediately" on his next move – if he makes any other move on his next move, he forever loses the chance to capture "that" pawn <WSD>"en passant</WSD>", though he still retains the right to capture "another" pawn <WSD>"en passant</WSD>" should a similar situation arise elsewhere. 
Meaning of the word marked with <WSD> tags:  Of a player's pawn when it has moved forward two squares on its first move in the game: captured "in passing" by the other player's pawn, as if the first player's pawn had only moved forward one square.