If a pawn reaches the eighth rank and chooses not to promote, can it go back the other way?
Imagine a White pawn going bxc8=P, then the unpromoted pawn moves to c7 and then that pawn gets stuck due a c6 White pawn. Both pawns cannot move. Even then, after several moves ahead, you may have already forgotten that the c6 pawn moves towards c1 by then.
promoting to pawn , pawn should be moved to the startting position !!