Pro Motion does have a large amount of shortcuts, but most artists will only need to learn the ones that they use the most in their workflow. Commands that aren't used that often are best accessed from the menu.
Here's a little guide to the keys I use and how I remember them:
Essential
Ctrl S - Save
LeftMouse - Paint with primary color
RightMouse - Paint with secondary color
Ctrl Z - Undo
Ctrl Shift Z - Redo
MouseWheel - Zoom in/out
Toolbox
D - Draw
AKA the pencil tool
F - Fill
AKA the paint bucket
V - Vertical line
Actually, this draws a line at any angle, but that's how I remember the shortcut.
B - Brush
This is what other apps refer to as the Rectangular Selection or Marquee tool. A brush is more powerful than a marquee because brushes can be stored in the brush container, thus allowing you to save multiple selections. You can also save a set of created brushes and use them in other projects.
. - Single pixel brush
If your brush looks funny after using the brush tool, press period to get back to the default brush.
Colors
[ and ] - go to the next or previous color in the palette
P - Palette Editor
Q - Gradient Editor
On a QWERTY keyboard, it's on the opposite side of the P key
Ctrl LeftMouse - Sample primary color
Ctrl RightMouse - Sample secondary color
Ctrl DoubleClickLeftMouse - Edit single color
Brushes
Shift - - Decrease brush size
Shift + - Increase brush size
H - Halve the brush Size
Shift H - Double the brush size
F2 - Paint mode
The regular painting mode. Paint modes are on the functions keys. F1 opens up help so F2 is the first mode key.
Shift F2 - Dither mode
The second most used paint mode.
F3 - Brighten mode
F4 - Darken mode
View
Shift S - Stencil Enable/disable
Shift T - Tweak the stencil
G - Grid enable/disabled
Enables grid snapping, tile drawing, or mirroring. RightClick the icon in the toolbox to change the settings.
Learning these keys will probably take a week or two at the most. As with most learned skills, practice makes perfect.