Beginner

Singles

Every solving method on this site is a machine for producing singles. A naked single is a cell with only one candidate left. A hidden single is a digit that can go in only one cell of a row, column, or box. Both are placements, not eliminations, and both are legal on the easiest puzzles.

Cages manufacture naked singles in the most obvious way: the last empty cell of a cage is the cage sum minus the digits already written. Check that the digit is still allowed in the row, column, and box. If it is not, an earlier digit is wrong. The last-cell rule does not get to break sudoku.

Hidden singles are how unique combinations pay off. After a sum-17 cage has removed everything but 8 and 9 from its two cells, look along each of their rows. If 8 is already impossible in every other cell of one row, that cell is 8 and its partner is 9. You did not need a new idea. You needed to notice that the cage had already done the deleting.

Pencil marks make singles visible and everything else noisy. On a phone, notes mode is the pencil. Use it for the candidates a cage still allows, not for every digit from 1 to 9. A cell with seven pencil marks is a cell you do not understand yet. A cell with two is a cell that is about to become a single or a pair.

When the hint button highlights a single, it will say which house or which cage forced it. Read that sentence before you tap the hint a second time. The second tap fills the digit, and the sentence is the part worth keeping for the next puzzle.

If you are brand new, play easy puzzles and refuse to place a digit unless you can name the single: last cell in a cage, only candidate in a cell, or only home for a digit in a house. That discipline is the whole beginner game. The combinations and the 45 rule exist to create those moments.

Use this technique on a puzzle

Keep the combination table open while you read, and the rules if the cages themselves are still new.