How to get better at Sudoku

Understand what a valid move is

Every Sudoku cell is defended by three constraints at once: the row, the column and the 3×3 box it sits in. A digit is only legal in a cell if it appears in none of the three. Internalising that triangle is the difference between scanning and guessing — you are looking for the intersection, not filling the first gap you see.

Start with singles, in the right order

A naked single is a cell where eight of the nine digits are already eliminated, so exactly one can go there. A hidden single is a cell that is one of several candidates, but where a digit can fit in only one cell of a row, column or box. Both are forced moves, and forced moves are free.

Scan for hidden singles before naked ones when a grid looks stuck. They are easier to miss and more common than people expect, and a puzzle that seems to require a guess usually still has a hidden single in it.

• Sweep one digit across the whole board at a time before moving to the next.
• Check every box for a digit that can only land in one square.
• When you place a digit, re-check its row, column and box — placements create new singles.

Keep pencil marks disciplined, or keep none at all

Full pencil marks are powerful and dangerous. Writing every candidate in every cell turns a puzzle into a wall of digits you have to read every time you scan, which is slower than the puzzle deserves. A workable middle ground: pencil-mark only the cells you are actively reasoning about, and erase a mark the moment it becomes impossible.

The worst habit is leaving a stale mark in place after the situation changed. A wrong note is worse than no note, because it makes a forced move look unavailable.

Prefer a slow correct move to a fast guess

Guessing is not just risky — it is expensive. A wrong guess can sit in the grid for several minutes before you notice, and by then you have built reasoning on top of it. If no forced move exists, look harder rather than writing a digit you cannot justify; there is almost always one more elimination to make.

This is the habit that transfers directly into Sudoku Duel, where the server generates one puzzle and both seats solve the same grid simultaneously with no time limit. Because there is no clock to force your hand, the player who stays on forced moves and never has to back out of a mistake almost always finishes first — patience is the faster strategy, which surprises people who expect a solving race to reward speed above everything.

A short daily routine

Solve one puzzle a day at the hardest level you can complete without guessing, and after each one, note the single moment you found hardest. That note is your training plan: tomorrow, look for that pattern first. Improvement in Sudoku comes from recognising more patterns, and you only recognise the ones you have deliberately looked for.