Common Sudoku mistakes

When a puzzle stops making sense, check the current entries before searching for a more advanced move. A contradiction can come from a placement or from an incorrect candidate note.

Checking only part of a cell's surroundings

A digit must satisfy its row, column and box together. Trace all three before confirming it. The rules page explains those groups with a small example.

Treating a possible digit as a certain one

“It fits here” is not enough if other candidates remain. Keep it as a note until you can explain why it is forced. If you made an assumption, distinguish it from a deduction so you can retrace it.

Trusting incomplete notes

A cell with one written note is not necessarily a single if you never recorded all its possibilities. Recalculate the cell from the current grid before placing that note as an answer.

Leaving stale candidates behind

After entering a digit, review notes in its row, column and box. Do not remove that digit from unrelated cells merely because it has appeared elsewhere on the grid.

Continuing after a contradiction

Stop if a group contains a duplicate, a cell has no possible digit, or a group has nowhere left for a missing digit. Check your recent entries and candidate removals. On paper, a fresh blank grid can help you copy the original clues and only the entries you have rechecked.

Reading an answer for the wrong puzzle

When using a generated pack with answers, match the date and difficulty before comparing the solution. Different puzzles can have very similar starting clues.

Next step after checking

If the entries look consistent but you cannot find a deduction, use the stuck-puzzle guide. For smoother practice after you can finish, try the speed routine. Return to all Sudoku tips to choose another starting point.