Good Sudoku strategy is about proving each placement. Start with simple scans, use notes when the board slows down, and clean candidates after every move so the next safe digit is easier to see.
Beginner
Scanning
Scanning means checking rows, columns, and boxes with many filled cells before looking at harder areas.
Pick a digit from 1 to 9 and scan where it already appears.
Look inside one 3x3 box and cross out rows or columns where that digit cannot go.
Place the digit only when one empty cell remains possible.
Beginner
Naked singles
A naked single appears when a cell has only one legal candidate left after row, column, and box checks.
Write the realistic candidates for a difficult empty cell.
Remove candidates already present in the same row, column, or box.
If only one number remains, that number belongs in the cell.
Beginner
Hidden singles
A hidden single is the only place a digit can go inside a row, column, or box even when the cell has other candidates.
Choose one row, column, or box.
Check every empty cell for one target digit.
If the target digit can fit in only one cell, place it there.
Intermediate
Locked candidates
Locked candidates let you remove notes when a digit is confined to one row or column inside a 3x3 box.
Find a box where all possible spots for a digit sit on one row or column.
Remove that digit from the rest of the same row or column outside the box.
Re-scan nearby cells after the notes are cleaned up.
Intermediate
Candidate cleanup
Candidate cleanup is the habit of updating notes after every placement so the next logical move appears faster.
After placing a digit, remove it from all peer cells.
Check whether any peer cell now has a single candidate.
Repeat the cleanup before searching for a harder technique.
How to practice these strategies
Practice one technique at a time. For a quick session, open an easy puzzle and focus only on scanning. For a longer session, try medium Sudoku and keep notes clean after each placement.