Learn / Sudoku
Find hidden singles
A hidden single is a digit that can go in only one empty cell in a particular row, column, or box. That cell may have several candidates, but the unit has no other place for this digit. Check each missing digit across all empty cells in the unit. Do not confuse 'one place in this row' with 'one place in the whole board'.
A checked example
Row 1, column 1 takes 4. In row 1, this is the only cell that can contain 4.