Hard is where scanning stops being enough. Few digits are given, the obvious moves run out early, and the puzzle starts asking for real technique. Free, no sign-up, unlimited puzzles.
Hard suits players who already finish easy and medium grids without stalling and want the puzzle to push back. Expect to sit with a grid rather than clear it on the first pass.
Fewer given digits mean fewer cells that can be solved directly. You have to work from candidates rather than certainties, and the chain of reasoning before the next digit gets longer.
When nothing moves, switch from hunting digits to hunting constraints: pick one digit and mark every cell it could occupy. The pattern that eliminates a candidate is usually easier to see than the digit that goes in a cell.
Yes. Every difficulty is free and needs no account.
Yes. Every generated puzzle is checked for a unique solution, so it is always solvable by logic alone, never by guessing.
Use a hint to open one cell, or switch on notes and rebuild the candidates. The grid never requires a guess.