The (distant) opposition, zugzwang, reserving pawn moves, triangulating the king are important concepts. The rule of the "square' helps in counting moves when defending against a passed pawn. Pawn breakthroughs need accurate calculation - advance the unopposed pawn to create a passer. "Turning the flank". King in oppositon on the 6th rank in front of his pawn on the 5th against lone king on the 8th is a win (except the rook file). Critical squares of the pawn determine whether an invading king can capture it. Chess Endings for the practical player - by L.Pachman - is a good reference.