Sudoku: a short strategy guide
Five things that carry most of the weight, in the order you should learn them.
1. Scan, don't guess
Pick a number, not a cell. Take the 4s: sweep each band of three boxes and ask where a 4 can still legally sit. In many grids a full sweep of one digit places two or three numbers before you have looked at a single empty cell in isolation. Beginners look for empty cells; stronger players look for digits that are nearly placed.
2. Note only when you have to
Pencil marks are a memory aid, not a strategy. Filling every cell with nine candidates creates clutter and makes a simple grid feel hard. Write candidates only in cells where two or three possibilities survive a scan, and clear a note the moment it stops being true.
3. The two singles
A naked single is a cell with one legal digit. A hidden single is a digit that has exactly one legal home inside a row, column or box — the cell itself may have four candidates, but nothing else in that unit can hold this digit. Hidden singles are where most of the progress lies in a medium puzzle, and they are invisible unless you are scanning by digit.
4. When you are stuck, ask what must be true
Look for a pair of cells in one unit that can only hold the same two digits — then neither digit can appear anywhere else in that unit. Look for a digit restricted to one line inside a box: that line cannot hold the digit outside the box. These two eliminations unlock the majority of hard puzzles without any guessing.
5. Never guess on a paying board
A guess is a coin flip that costs you the board. If nothing follows logically, re-scan the units you have been ignoring — usually the boxes on the far side of your last placement — rather than committing to a number you cannot justify. The timer on our board is display-only, precisely so that pausing to think is never punished.
A note on difficulty
"Hard" here means the solver had to backtrack a lot, not that it needs an advanced technique from a book. If a board feels unfair, tell us — edmarpiedad@gmail.com — and include roughly when you started it, so we can find it.