Make your notes work for you
Turn a crowded grid into a set of smaller possibilities.
A candidate is a possibility.
A candidate is a digit not yet ruled out by the row, column, or box of an empty cell. It is not a prediction. A cell may start with five candidates and gradually narrow to one as you fill neighboring cells.
Add notes without changing the board.
Select an empty cell and turn Notes on. Choose a number to add a small candidate; choose it again to remove it. Turn Notes off to enter your answer. Candidate digits always keep the same positions inside the cell, making repeated patterns easier to see.
Maintain the possibilities.
When you place a number, the game removes that candidate from notes in the same row, column, and box. Undo restores both your entry and those notes. The game does not automatically fill every candidate for you, so incomplete notes must not be mistaken for proof.
Find a single.
If a correctly maintained cell has only one candidate left, place that digit. But first check that you did not accidentally leave out another possibility. The hint system calculates candidates from the actual board rather than trusting handwritten notes.
Then look for pairs.
If two cells in one unit can only contain the same two digits, those digits are reserved for those cells. You can remove both digits from the other cells in that unit. This is a naked pair, and it is a useful bridge toward harder puzzles.