What changes at hard
Scanning alone will not finish a hard grid. After the opening placements you will hit a wall where no cell has an obvious single, and the way through is to look at what candidates share cells: a naked pair, a hidden pair, or a triple that locks three digits into three cells.
A method that works
- Scan first. Even a hard grid gives up six to twelve cells to plain scanning.
- Mark candidates for one box or line at a time, starting with the unit that has the fewest empty cells.
- Look for two cells in a unit holding the same two candidates. Those two digits belong to those two cells, and can be struck out everywhere else in the unit.
- Check whether a digit is confined to one line inside a box before you reach for anything more advanced — locked candidates are cheap and they often unlock the rest.
- Keep your marks honest. One stale candidate will send you down a false path and cost you more time than the puzzle.
No guessing, ever
A hard puzzle here is still a logical puzzle. The generator refuses any grid it cannot solve with named techniques, so when you are stuck there is a real move to find. Press hint and the board will name it and point at it.
If you are finishing these comfortably, expert is where wings and fish start to matter.
Common questions
How many clues does a hard Sudoku have?
Usually 24 to 30. The difficulty comes from the techniques required, not only from the count.
Which techniques do I need for hard puzzles?
Scanning, locked candidates, and pairs or triples. Wings and fish are reserved for expert.
How long does a hard Sudoku take?
Fifteen to forty minutes is normal. Experienced solvers who mark candidates efficiently are often under ten.
Is there always a logical next move?
Yes. Every puzzle is verified before publication as solvable by technique alone, so you never have to guess and backtrack.