Sudoku: candidates, singles and safe progress
These examples explain deductions, not a pre-written solution to today's puzzle.
Write candidates from all three units
For an empty cell, start with 1–9 and remove digits present in its row, column and box. Recheck all three: a row-only check can miss a box conflict. Candidates are possibilities, not entries.
Naked single: one possibility in a cell
Example: the row excludes 1,2,3; the column excludes 4,5; and the box excludes 6,7,8. Only 9 remains. Enter 9, then recompute candidates for cells sharing those units.
Hidden single: one location for a digit
Suppose the empty cells in one row have candidates {2,7}, {2,4}, {4,7,9}. Digit 9 has only one possible location in that row, so the third cell is 9 even though it had multiple candidates. This example assumes the other row cells are already filled and the candidate sets include column and box restrictions.
When progress stops
Recheck candidates and inspect a different unit. If you try a guess, record it so you can undo it; a guess is not proof. The game's reveal button supplies a value from the unique solution and records that a cell was revealed. Use it when you want assistance, rather than treating it as an unexplained logical deduction.
Why check conflicts and answers separately?
A digit can fit the visible row, column and box while still leading to an impossible completed grid. Our Check button compares filled entries against the verified solution; this is assistance, not a candidate solver.