Hard means the puzzle cannot be finished with singles, unique cages, and one-cell 45 steps alone. You will need a two-cell innie or outie, a cage whose digits are locked by overlapping combinations, a split where a cage crosses a box, or a triple. You will need that kind of step at least three times. A puzzle that is gentle except for one nasty cell is not hard, and it is not in this set. It is a medium puzzle with a bad cage, and we do not publish those.
There are no given digits. Cage counts sit lower than medium, so the sums are wider and the combination lists start longer. The offset is that the 45 rule has more to say, because more cages cross a border. Your first five minutes should still place digits. If they do not, look for the smallest outie before you look for the largest cage.
The failure mode on hard is inventing a bifurcation: “suppose this cell is 6.” The puzzle does not need it. If you are about to suppose, ask for a hint instead and read which cells it lights up. The second request fills a digit, but the first request is the one that tells you the name of the technique you skipped. Repeating that name on the next hard puzzle is how the set gets faster.
Expect twenty to thirty-five minutes. The clock pauses when you pause, and the board is covered so a break is a break. Your best time is stored on this device for that exact puzzle number, which makes a rematch meaningful in a way a random generator is not.
If hard is comfortable, expert is the same family of techniques with a higher bill: more of them, in a tighter cage layout, and a longer sit. If hard is miserable, the missing piece is almost always innies and outies as pairs, not as single cells. Read that page, then come back to the same puzzle number. It will still be here.
Related reading: how to play, the 45 rule, and the combination table.