The moment a puzzle turns Hard
You fill in singles for a few minutes, the grid half full, and then nothing: every empty cell still has two or three candidates and no digit is down to one place. On an Easy or Medium grid that never happens. On a Hard grid it happens about three times per puzzle, and each time one small pattern unlocks the next run of singles.
So the skill at this level isn’t looking harder at the board, it’s reading your pencil marks. Turn on Notes and tap Fill notes: every candidate goes in at once, and each digit you place clears itself from the notes it rules out, so the marks stay correct as you go.
Four questions to ask your notes
Does a digit’s notes inside one box all sit on one line? Say the only 6s left in the middle box are both in row 5. Row 5’s 6 must be one of those two, so every 6 note in row 5 outside that box can go. This one pattern is behind most Hard puzzles here.
Does a digit’s notes along one line all sit in one box? The mirror image: if column 2’s only 9s are inside the top-left box, the other cells of that box lose their 9 notes.
Do two cells in a unit hold the same two notes and nothing else? Those two cells will take exactly those two digits, whatever order they come in, so strike both digits from the rest of the unit.
Do two digits appear in only the same two cells of a unit? Then those cells belong to them; wipe every other note from the pair.
A routine
Place every single first. Then check each box for the first question, digit by digit, 1 to 9: it’s quick and it pays off most often. Only then look for twins. After any elimination, look for singles again before hunting for the next pattern. If you’ve checked everything, Hint highlights the cells and shows the chain from pattern to digit.
Hard is also the daily puzzle on Thursdays, Fridays and Sundays.