Print today's six hard Sudokus with their answer key. These grids start with fewer numbers and need subset techniques, naked and hidden pairs and sometimes triples, to clear candidates when no single is in sight. Each has one solution, reachable without guessing.
Grid #1 is today's hard grid. Play it here with the coach for step-by-step hints (unranked; the ranked Daily Sudoku is expert today).
For hard grids it's worth writing every candidate. Once the obvious singles are in, go through each empty cell and write the digits that could still go there, small and in the same position each time (1 top left, 9 bottom right) so patterns jump out. Then look in each row, column and box for two cells holding the same two candidates: that's a naked pair, and those two digits can be erased from the rest of the unit.
A hidden pair is the mirror image: two digits that can only go in the same two cells of a unit. Every other candidate in those two cells can go. Print one grid to a page if you write full notes; the larger cells keep them readable.
Check your candidates before hunting for something clever: a missing candidate is the most common reason a hard grid seems to stall. If you want a nudge without the answer, play the first grid on this sheet online, where the coach can point to the next step one hint at a time.
They need pairs and sometimes triples: steps where you reason about two or three cells together to rule digits out. No X-Wings and never any guessing.
Usually 20 to 45 minutes on paper, longer if you're new to pairs.
Grid 1 on the sheet is also playable on this page, with a coach that explains the next step. For the others, the answer key shows the full solution.
Yes. Every grid is checked by a solver that only uses logic; finishing by logic proves there is exactly one solution.