Sudoku is a logic puzzle with one goal: each row, column, and 3×3 box must contain the digits 1–9 exactly once. A reliable solving routine reduces guesswork by keeping the candidate list current.
Scan rows, columns, and boxes for obvious placements. When that stalls, write the legal candidates in each empty cell. A naked single has only one candidate; a hidden single is the only place for a digit within a row, column, or box. After every placement, remove that digit from related candidates before searching again.
If two cells in one unit contain the same two candidates, those digits can be removed from the other cells in that unit. Check the pair really has exactly two candidates in both cells. If you are still stuck, revisit stale notes before guessing.
Practice a fresh grid at Sudoku.
Akousa