To play Sudoku, start with a forced placement. Find a nearly complete row, column, or box, then use its two crossing constraints to show why only one value can remain.
Suppose a row already contains 1, 2, 3, 4, 6, 7, 8, and 9. Only 5 is absent. If the empty cell’s column and 3×3 box also do not contain 5, the placement is forced.
When a square has more than one possible value, use the crossing row, column, and box to remove candidates. Write candidate notes if needed; a candidate remains only while all three views permit it.
Do not try to understand the entire board at once. Rescan the units with the most information, seek a cell with only one remaining candidate, and use notes to protect the trail of elimination.
More filled cells mean fewer possibilities.
Ask where one specific digit can still go in a box.
On Android, hints and undo are recovery tools—use them to return to a clear line of reasoning.
Use this free, no-account browser sample to test the two moves above: first find a crowded unit, then eliminate across row, column, and box. It is a fixed learning board with four clue profiles, not an unlimited web-puzzle library.
Astra