Singles
A single is a digit you can place because nothing else is legal. You do not compare it with a guess. There are two kinds. Both use the same row, column, and box rules.
Naked single
A naked single is a cell with only one candidate left. Suppose one empty cell sees 1, 2, 3, 5, 6, 7, 8, and 9 already used by its row, column, or box. The only digit not used is 4. Place 4. If your notes in that cell are only “4”, that is the same situation: the notes have already done the elimination.
Hidden single
A hidden single is about the digit, not the cell. Look along one row and ask where 4 is still allowed. If eight cells of the row cannot take a 4, the ninth cell must be 4, even when that cell’s own notes also list 2 and 7.
Worked row, left to right. Filled cells are shown as digits. Empty cells are shown as the candidates you would have written with Notes:
- Cell 1 is filled: 8
- Cell 2 notes: 2, 4, 7
- Cell 3 is filled: 1
- Cell 4 notes: 2, 7
- Cell 5 is filled: 6
- Cell 6 notes: 3, 9
- Cell 7 is filled: 5
- Cell 8 notes: 3, 9
- Cell 9 notes: 3, 7
In that row, 4 appears in the notes of cell 2 and nowhere else. Cell 2 also lists 2 and 7, so it does not look like a naked single. The 4 is still forced: no other cell in the row can hold it. Place 4 in cell 2, and delete 2 and 7 from that cell. Then check the column and the box that contain cell 2, and remove 4 from their notes.
The same scan works down a column and inside one box. If you cannot find a single, stop and mark candidates instead of trying a digit that still has two homes. Notes is the tool for that.