Learn / Sudoku
Common Sudoku mistakes and how to recover
Do not change the original clues. Check boxes as well as rows and columns. Treat notes as possibilities, not answers. Update candidates after every confirmed placement. If you hit a contradiction, undo the earliest uncertain answer and recompute the notes it affected. Take a break when repeated scanning stops producing progress; a fresh pass may reveal a single you missed.
A checked example
Row 1, column 7 takes 3. Every other digit is already excluded by its row, column, or box.