Stuck on Sudoku? Nine Logical Things to Try

Feeling stuck usually means the next useful fact is smaller than the pattern you are searching for. The temptation is to stare harder at the whole board or test a candidate. Both increase noise.

Instead, move through this checklist in order. Each pass narrows your attention and gives you a clear reason either to place a number or continue.

1–2. Return to the last change

Recheck the row, column and box touched by your most recent placement. Then inspect any unit with seven or eight filled cells. A number that looked unhelpful before the last move may now be a naked or hidden single. This small reset catches many apparently stalled easy and medium puzzles.

3–4. Search by digit, then by cell

Choose a digit that appears often and trace where it can still fit in every box. Afterwards reverse the question: choose an empty cell and list what its three units permit. Switching between “Where can this digit go?” and “What can this cell contain?” reveals different kinds of single.

5–6. Audit your pencil marks

Check a few promising cells against their complete row, column and box. Remove candidates blocked by later placements and add any candidate you accidentally omitted. Then look for two cells in one unit that share the same pair; those digits are reserved there and can be removed from other cells in that unit.

7–8. Check box-line interactions

Within each 3×3 box, look for a candidate confined to one row or column. That candidate can be removed from the rest of the same line outside the box. Also check the reverse: if a candidate in a row can only sit inside one box, remove it from the other cells of that box.

9. Use one hint as an explanation

If the puzzle still does not move, request one hint rather than several. Before entering anything else, inspect the hinted cell and prove why its number is forced. A hint becomes instruction when you can identify the blocked candidates or pattern that produced it.