Normal Operation

Using the mouse (normal operation)
Clicking a cell selects that cell. Pressing [ctrl] or [shift] while clicking selects a region of cells. Setting cells/candidates:
Using the mouse (alternate mouse mode)
Region select as described above. Setting cells/candidates:
Using the mouse (coloring mode)
Clicking a color in the status bar or the Active Cell view activates coloring mode. Clicking a cell/candidate toggles the selected color for that cell/candidate. Pressing [shift] while clicking uses another color. Clicking the background color or pressing [esc] ends coloring mode. Clicking the panel with the "R" ends coloring mode and deletes all colors.
[left], [up], [right], [down]
Moves the cursor (the highlighted cell).
[shift][left], [shift][up], [shift][right], [shift][down]
Expands the range of selected cells.
[shift][ctrl][left], [shift][ctrl][up], [shift][ctrl][right], [shift][ctrl][down]
If filters are applied, the cursor is moved to the next unsolved cell, that contains the filtered candidate.
[ctrl][left], [ctrl][up], [ctrl][right], [ctrl][down]
Moves the cursor to the next unsolved cell.
[home]
Moves the cursor to the leftmost column.
[end]
Moves the cursor to the rightmost column.
[ctrl][home]
Moves the cursor to the topmost row.
[ctrl][end]
Moves the cursor to the bottommost row.
[1] ... [9]
Sets the value in the highlighted cell; givens (shown in black) cannot be overridden.
[Ctrl][1] ... [Ctrl][9]
Toggles the candidate in the highlighted cell.
[blank]
If a filter is set (see below) toggles the filtered candidate.
[del], [backspace], [0]
Deletes value in cell (givens cannot be deleted).

Filters

Filters come in two varieties: Show all allowed cells for placing a certain digit (cells are green) or show all forbidden cells for a digit (cells are red).

[F1] ... [F9]
Sets/toggles the filtered digit.
[shift][F1] ... [shift][F9]
Sets/toggles the filtered digit and switches the filter mode (allowed/forbidden).
[ctrl][F1] ... [ctrl][F9]
Adds/removes the candidate to/from the set of filtered candidates. If more than one candidate is selected for filtering, either all cells are shown, that contain at least one filtered candidate ('OR'), or only cells are shown, that contain all filtered candidates ('AND').
[space]
If a filter is active and the current cell is filtered, toggle the filtered candidate in the cell.
[enter]
If a filter is active and the current cell is filtered, set the filtered candidate in the cell.
[>], [.]
If a filter is active, set the next digit.
[<], [,]
If a filter is active, set the previous digit.

Coloring

[a]/[A], [b]/[B], [c]/[C], [d]/[D], [e]/[E]
Sets/toggles background color of the selected cell.
[r]
Resets background on all cells.

Miscellaneous

[shift][ctrl]
Show all possible candidates in cell with focus (only if "Options|Show all candidates" is not set).
[shift][alt]
Show all possible candidates in all cells (only if "Options|Show all candidates" is not set).

Menu and Dialogs

[ctrl][n]
Create new random sudoku.
[ctrl][o]
Open puzzle file.
[ctrl][s]
Save sudoku.
[ctrl][p]
Print sudoku.
[alt][x]
Quit program.
[ctrl][z]
Undo.
[ctrl][y]
Redo.
[ctrl][c]
Copy candidates to clipboard.
[ctrl][g]
Copy givens to clipboard.
[ctrl][v]
Paste sudoku from clipboard.
[ctrl][r]
Restart game.
[ctrl][shift][p]
Edit Preferences.
[ctrl][shift][1] ... [ctrl][shift][5]
Set difficulty level for newly created sudokus (1 ... Easy to 5 .. Extreme).
[alt][F12]
Show vage hint.
[ctrl][F12]
Show concrete hint.
[F12]
Show next step.
[F11]
Set all singles.
[ctrl][shift][s]
Show sudoku only.
[ctrl][shift][u]
Show summary of steps used to solve the sudoku.
[ctrl][shift][o]
Show solution(s).
[ctrl][shift][a]
Show all possible steps for current state.