Hard Sudoku

Slow down and work the notes

Hard puzzles start sparse, so the opening feels stuck on purpose. The way through is discipline: pencil in every candidate for the empty cells first, then hunt for the small contradictions — a digit forced into one cell, a pair that locks two cells, a candidate that can only live in one line of a box.

These grids reward returning to the same region twice. A mark you couldn't use ten moves ago often becomes the key once a neighbouring box fills in. There's never a need to guess — every hard board here still has exactly one solution reachable by logic.

If a hard puzzle stalls, that's the signal to re-pencil a box you filled early — the answer is usually a candidate you cleared too soon, not a leap you haven't taken.