Solving Sudoku becomes easier when you use observation methods instead of random guessing. Below are useful strategies for beginners and intermediate players.
1. Missing numbers
Choose a row, column, or box with many filled numbers. Mentally list the missing numbers and check where each one can go.
2. Single candidate
When a cell can only take one possible number, it should be filled in. This is one of the most important Sudoku techniques.
3. Unique position in a box
Sometimes a number can appear in only one position inside a 3x3 box. Even if that cell has other candidates, that number is fixed.
4. Use notes wisely
Notes help, but too many marks can confuse you. Use them mainly in medium and hard puzzles.