Back hundreds of years ago, pawns could only move once forward, even on their first go. This was found to make for very slow openings, and so the ability to move a pawn forward two on its first move was introduced. In addition, a rule known as en passant ('in passing') was introduced- if a pawn moves forward twice, then on the subsequent move, if available, an opposing pawn may capture it as though it had only moved forwards once.
NOTE: This can only be done on the move DIRECTLY after the double-pawn move; not later.
Playing today online (I was white) my oponent made a move that seemed to me very ilogical:
18. a4 bxa3
I couldn't understand how a pawn already passed could eliminate another. My pawn was in paralel to the black one, nevertheless it took mine and went directly to the a3 square...how???
At first I thought it was a bug...but checking on the net, I have found hundred of games with the same movement.
So, from my deep ignorance, I've got a question...is that a legal movement? Could anyone explain what is the rule that enable this weird movement?
Thanks in advance for bringing some light to this matter.