Updated June 10th, 2019.
You know that you can press Tab on the keyboard and move to the next field in a form, or the next cell in a spreadsheet, or the next part of a Print… box etc. Yes you knew that. But did you know that holding the Shift key down while you press Tab moves you to the previous field, cell, part, etc.?
You know that ⌘-Z is “Undo.” And if you Undo something, and want to redo it… what do you do? Add the Shift key. ⌘-Shift-Z is “Redo.” Doesn’t work everywhere but it’s worth a try.