Pick a difficulty, enter numbers with keyboard or touch, and print the current puzzle whenever you want.
A hard Sudoku starts with only about 30 clues, leaving many cells with several possible numbers. Plain scanning is not enough anymore. This level is for experienced players who combine clean logic with notes and can follow longer chains of reasoning. A hard puzzle rewards focus and often takes fifteen to thirty minutes; guessing almost always leads to a dead end.
A Sudoku does not become hard through bigger numbers but through fewer clues and longer logical chains. You often have to consider several cells at once before a safe move appears. That is exactly the appeal: every bottleneck you clear opens up several new possibilities.
On the hard level you need advanced patterns. In a Naked Pair, two cells in the same unit allow only the same two numbers, which can then be removed everywhere else in that unit. The same idea with three numbers is a Naked Triple. These techniques crack positions where singles no longer help.
When pairs are not enough, the X-Wing pattern often helps. You use two rows in which a number is possible only in the same two columns to eliminate that number elsewhere in those columns. X-Wing is the first step into advanced Sudoku logic and prepares you for the expert level.
Start with an easy Sudoku. Focus first on rows, columns and 3x3 boxes that already contain many digits, because those usually reveal the next safe move fastest.
Move to another area of the grid and compare row, column and box again. It also helps to use notes for candidates instead of guessing.
Easy puzzles start with more given numbers and shorter logic chains. Harder levels provide fewer clues, so solving them requires more careful deduction and longer sequences of reasoning.