Original: When a pawn advances two squares and there is an opponent 's pawn on an adjacent file next to its destination square , then the opponent 's pawn can capture it en passant ( in passing ) , and move to the square the pawn passed over .
Simplified: En passant ( ` in passing ' in French ) is a special capture . It is only available when a pawn moves forward two squares past an opposing pawn on an adjacent file .