Follow the possibilities. A small connection can open up the board.
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Choose an empty square, then a number. You’ve got this.
Keyboard friendly 1–9 numbers N notes ⌫ erase ← ↑ ↓ → move
Classic Sudoku rules Fill every row, column and 3×3 box with the digits 1–9, using each digit exactly once. The given numbers stay fixed. Start with an empty square and check all three groups.
Our medium boards require locked candidates as well as singles. You will sometimes reach a position with no immediate number to place. At that point, looking at where a digit can go across a box and a row or column can uncover an elimination.
Suppose every possible 7 in a box lies along one row. The box must contain a 7, so that row will get its 7 inside the box. You can remove 7 from other empty cells along the same row outside the box. This may create an only candidate or an only place.
Pencil marks make candidate patterns easier to compare. Automatic notes show the candidates allowed by the currently filled digits. Logical eliminations can go further than those basic candidates. The hint panel explains its eliminations before showing a placement, so you can follow why the step works.