Learn Sudoku: rules, pencil notes, and explained hints
Sudoku is a logic puzzle, not arithmetic. Each number needs a place where it is not repeated in the same row, column, or box.
Published by Mind-Sprout · Responsible for content: Max Hafer
Revised:
How this guide is prepared
This guide was revised with AI assistance. Practical instructions refer to Mind-Sprout features; research statements link to their sources. Please report corrections using the contact details in our legal notice.
The three rules
A 9×9 Sudoku uses 1 to 9 once in each row, column, and 3×3 box. Given numbers cannot change. Mind-Sprout also offers 4×4 puzzles using 1–4 with 2×2 boxes, and 6×6 puzzles using 1–6 with boxes two rows high and three columns wide.
Find a missing number
Start with a row, column, or box that already has many numbers. If a 4×4 row is missing only 2 and 4, also check each empty cell’s column and box. If either already contains 2, that cell must take 4.
Leave uncertain cells empty. An entry that produces no immediate conflict is not necessarily correct; its consequences may only appear later.
Keep candidates in note mode
Choose an empty cell and turn on “Note mode”. Tap possible numbers; tap again to remove a note. N also toggles the mode on a keyboard. Notes are your own possibilities and do not count as filled cells.
Return to number mode when a value is certain. Entering it clears the cell’s notes and removes matching notes from its row, column, and box. Undo restores previous notes as well as numbers.
Understand a hint
“Explain a hint” lists candidates for your selected cell or explains why one value remains. Without a selection, it looks for a simple next step. It reasons from visible entries; incorrect entries can lead to an incorrect conclusion.
A definite number is entered only when you choose “Apply”. This uses one of the limited assisted entries. You can view explanations as often as you like. If simple rules cannot determine a value, the explanation gives candidates instead of guessing.
Play, pause, and continue
Arrow keys move focus. Number keys enter values or notes; Delete and Backspace clear a cell and its notes. Library puzzles save progress, notes, and undo history in the browser on the same device. Clearing browser data removes the saved game.
After completion, follow the link to a Sudoku of the same size and difficulty. Choose Print for a paper worksheet with an optional answer page. Sudoku here is a leisure and learning activity, not a medical training programme.