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Tap a cell, then tap a digit on the pad. The small number on each pad key shows how many of that digit are left to place.

Turn on notes mode on the pad to pencil in candidates. You can also long-press a digit key to use the other mode for a single entry. Notes are cleared automatically when you place a digit that rules them out.

Hints are unlimited and free. The first tap marks a cell where a logical next move exists; the second tap fills in the digit.

By default the app flags a digit that doesn't match the solution. If you'd rather find mistakes yourself, turn off strict checking in Settings; then only direct conflicts in a row, column or box are highlighted.

Never. Every puzzle has exactly one solution and can be solved by logic alone, including Expert. Difficulty comes from the techniques a puzzle needs, not from how many clues it has.

Yes. Your current game is saved on your device and resumes where you left off. The timer pauses whenever you leave the app. Starting a new game replaces an unfinished one, and the app asks before it does.

Completely. Ensō Sudoku never needs a connection.

Yes, both have switches in Settings.