Scan before you write
Good solving starts with looking. Pick a digit and sweep across the rows and columns that already contain it. Any row or column with that digit rules out every other cell in that line. Boxes that still lack the digit often have only one open cell left.
Naked singles
A naked single is an empty cell that can hold only one digit. Look at the row, column and box that cell belongs to. If eight of the nine digits already appear in those three places, the ninth is the answer.
Cells in crowded rows, columns and boxes are the best place to look first.
Hidden singles
A hidden single is a digit that has only one possible cell within a row, column or box, even if that cell could hold other digits too. Pick a box, choose a missing digit, and check each empty cell in the box against that digit. If only one cell survives, place it there.
Easy and medium puzzles on this site can be solved with naked and hidden singles alone.
Pencil marks
When scanning stops producing digits, write small candidates in the corners of empty cells. Use a light pencil so you can erase cleanly. Marks are most useful in cells with two or three candidates. Filling every cell with marks at the start is slow and makes the grid harder to read.
- Work box by box: list candidates for one box, then move to the next.
- When you place a digit, erase that digit from the marks in its row, column and box.
- Look for a digit that appears as a candidate in only one cell of a unit.
Avoid guessing
A guess may work for a few moves and then fall apart, and finding the original mistake is tedious. Every puzzle here has one solution that logic can reach, so if you feel stuck, rest your eyes, switch to a different digit or box, and rescan. Erase a mark or a digit as soon as you find a conflict, and trace back to the last point where you were sure.
Have a correction or a suggestion? Tell the studio.