Four techniques solve almost every sudoku: scan one digit at a time across the grid, place naked singles (cells with only one remaining candidate), find hidden singles (digits with only one possible home in a row, column or box), and keep pencil marks current so the harder patterns reveal themselves.
Every sudoku can be solved by logic alone — the difference between difficulties is which logic. These four techniques, learned in order, carry you from your first easy grid to expert territory. (New to the rules? Start with how to play.)
1 · SCANNING
Take one digit — say 7 — and sweep the grid. Every placed 7 forbids its whole row and column, so in many boxes only one cell can still take a 7. This 'cross-hatching' fills surprisingly much of an easy grid before you think hard at all.
2 · NAKED SINGLES
Look at one cell and ask what it can still be. If its row, column and box already contain eight different digits, only one candidate is left — a naked single. It's forced; place it and ride the chain reaction.
3 · HIDDEN SINGLES
Flip the question: inside one box (or row, or column), where can the 4 go? If every cell but one is blocked, the 4 is hidden there — even when that cell has other candidates written in. Hidden singles crack most medium grids.
4 · PENCIL-MARK DISCIPLINE
From hard difficulty up, keep candidates written in every empty cell (auto-notes does the initial sweep here). The advanced patterns — pairs, triples, pointing candidates — are all things you notice in the pencil marks, not in the big numbers.
When a grid stalls
Stuck grids are almost never actually stuck — a candidate somewhere is wrong or missing. Re-scan your most recent placements before hunting exotic patterns; nine times out of ten a plain single was hiding behind a stale pencil mark. If you'd rather keep momentum, a hint here points out a cell that has exactly one candidate — it teaches the same habit the techniques do.
Techniques, multiplied
Co-op mode is quietly the best trainer: one player scans while another chases singles, and you see each other's reasoning land on the shared grid in real time. Bad candidates get caught faster with four eyes on the board.