Sudoku mastery is candidate management. Every strong solve is built on eliminating impossibilities until only valid placements remain.
Candidates are digits that can legally fit in a cell based on row, column, and box constraints.
1) Direct elimination (row/col/box)
2) Cross-constraint elimination (box-line interactions)
3) Group elimination (pairs/triples)
4) Cascading elimination (chain reactions)
Placement is the result; elimination is the process. Clean candidate elimination makes patterns visible and turns “stuck” boards into solvable ones.
Candidate notes should reduce mental load, not increase it. Start with one unit that looks promising, such as a box with four empty cells, instead of filling the entire board immediately.
After every placement, remove affected candidates from the same row, column, and box. This maintenance step is what turns notes into a solving tool. Without cleanup, notes become stale and can lead to false conclusions.
If you cannot explain why a candidate was removed, do not remove it yet. Good elimination is reversible in your reasoning: you should be able to point to the row, column, box, or pattern that made the candidate impossible.
Suppose a cell can only be 2, 5, or 8 after checking its row, column, and box. Write those candidates only if the cell matters to the current area you are solving. If another cell in the same row later becomes fixed as 5, remove 5 from this cell. The note changes from 2-5-8 to 2-8.
This is simple, but it is the heart of candidate management. Notes are not static labels. They are working information that must be updated when the board changes.
Full-board notes can help in difficult puzzles, but they can overwhelm beginners. A better intermediate approach is focused notation: choose one row, column, or box with several constraints and write candidates only there. Solve or simplify that area, then expand outward.
Before removing a candidate, name the reason. Was it blocked by a given? Removed by a pair? Limited by a box-line interaction? If you cannot name the reason, keep the candidate for now. Conservative notes are safer than overconfident removals.