Easy shows each number you place as right (green) or wrong (red). Hard only flags rule-breaks in red - the rest is up to you.
The numbers form one path: 1 sits next to 2, 2 next to 3, and so on up to 25 - up, down, left, or right. Fill each empty cell with the number that links its neighbours in order (between a 9 and a 12, the gap is 10 then 11). Wrong moves flash red; it auto-completes when the grid is correct.