In standard Sudoku, candidates are digits still allowed by a cell's row, column and 3×3 box. Pencil marks record possibilities, not confirmed answers.
Two different ways a digit becomes forced
A naked single is a cell with one candidate. A hidden single is a digit that has only one possible cell within a row, column or box. For example, if a row is missing 2, 5 and 8, with candidates {2,5}, {2,5} and {5,8}, the third cell must be 8 because no other empty cell accepts it. The first two cells still need more information.
Try the steps
- List the digits missing from one row, then remove candidates forbidden by each column and box.
- Check both cells with one candidate and digits appearing in only one candidate set in that unit.
- After placing a digit, update notes in its row, column and box before making another deduction.
Should I enter a number just because it appears in my notes?
No. A candidate means possible, not proven. If a cell has no candidates, check earlier entries and stale notes. Use the game's note mode if provided, or paper otherwise. These steps concern standard 9×9 Sudoku; variants may introduce extra constraints.