Stuck on Sudoku? A step-by-step checklist before guessing
Check mistakes, repair notes and scan the simplest patterns in a useful order. Know when a hint is helping and when it is only revealing an answer.
First check the position, not the difficulty label
A stalled puzzle does not immediately require an advanced technique. One incorrect entry or missing note can hide several simple moves. First inspect your recent placements and explain why each was forced.
An entry can be wrong even when it creates no visible repeat. Passing the row, column and box checks only means a digit is currently allowed; it does not prove that digit belongs in the final solution.
Make the notes trustworthy
Choose an area where progress stopped. For each empty cell, compare its notes against the digits in the row, column and box. Remove candidates blocked by placed digits and restore any candidate you accidentally omitted.
Preserve eliminations justified by a pair or another proved pattern. If you cannot reconstruct an elimination, return to an earlier reliable state. On iPlay Sudoku, Undo restores the previous move so that you can recheck its effect.
Scan in this order
Use a repeatable routine instead of repeatedly looking at the most empty part of the board.
- Naked singles: does any cell have exactly one candidate?
- Hidden singles: does any missing digit have only one place in a row, column or box?
- Locked candidates: are all positions for a digit confined to a box-line overlap?
- Naked pairs: do two cells in one region contain the same two candidates?
- After each elimination, return to singles.
Separate a useful hint from an answer reveal
A useful hint explains which constraints force a move. Read the explanation, identify the relevant cells, and try to state the reason yourself before filling the answer.
iPlay Sudoku first looks for a wrong entry or a single. If it cannot find one, its hint may reveal a solution digit. That reveal does not mean the puzzle has no logical next step, and it is not an explanation of an advanced technique. Use it deliberately if you want to continue, or pause and inspect a technique guide.
A quick diagnosis
Do not confuse the number of notes in a cell with the number of positions for a digit.
One box has three empty cells with candidates {1, 4}, {4, 7}, and {4, 7}. Which move is certain?
Show the explanation
The first cell is 1: it is the only possible position for 1 in the box, a hidden single. You can also see the pair {4, 7} in the other two cells, which removes 4 from the first cell. Two explanations reach the same move.
Choose a useful practice goal
If you are learning pairs, aim to explain one pair correctly rather than to finish quickly. Difficulty labels are relative to each puzzle service; fewer starting digits do not by themselves prove a puzzle is harder.
Take a break when you are scanning without noticing changes. Return to the same saved board with one question, such as “Where can 6 go in this box?” You can make progress without setting a speed target.