Singles

Look for a row with only one empty cell left. Then find out what digit is missing in that row and fill the empty cell with it. That’s it. The same works for columns and 3 x 3 blocks. In the first example below you can place digit 9 in row 7. In the second example you can place digit 6 in block 8.

Select a 3 x 3 block and a digit. Check how many cells in that block can contain the selected digit. If there is only one such cell, you can fill that cell with the selected digit. The same logic is applicable for rows and columns. A cell can contain a digit if there is no such digit in the same row, column and 3 x 3 block.

Select a cell and check how many digits are allowed in that cell. If there is only one allowed digit, you can fill the cell with that digit. A digit is allowed in cell if there is no such digit in the same row, column and 3 x 3 block. In the first example below the highlighted cell can contain only digit 3. In the second example - only digit 4.