Audit for "Ghost Singles" (The Most Common Blindspot)
When you place a digit earlier in the game, it's very easy to forget to erase that digit from the candidate notes of neighboring cells in that row, column, or block.
Look for Hidden Singles (Scan by Digit, Not by Cell)
A cell might have 3 or 4 candidate notes inside it, but within its 3x3 block, row, or column, one of those notes can appear nowhere else.
Check for Pointing Pairs (Box-Line Reduction)
Inside any 3x3 box, if all instances of a candidate digit fall along the same row or column, that digit is "locked" in that line.
Spot Naked and Hidden Pairs
If two cells in the same row, column, or block contain strictly the same two candidates (for example, [3, 8] and [3, 8]), no other cell in that unit can be a 3 or an 8.
Hunt for Geometric Fish Patterns (X-Wing & Swordfish)
When subsets fail, the puzzle requires multi-line geometric locking. Look for digits that appear only twice across two parallel rows or columns:
Deploy Bivalue Forcing Chains (AIC)
In diabolical and evil puzzles (like the NYT Hard Sudoku), you must trace consequence branches: "If cell A is 4, then cell B must be 9, which forces cell C to be 2..."