FCPX command modifiers

I am trying to get behind the logical system (if it exist) when and how to use the modifiers (Shift, Control, Option and Command) in combination with a keyboard letter to execute a certain command in FCPX.
Does anyone has any pointers?
Maybe some easy way to remember, when to use the command or option etc.

There's no real logic. Just learn each command as you need them. When I do something more than one or two times during an edit session, I look at the menu for the shortcut, and force myself to use it. If I forget it, I make myself recheck it in the menu and use it. After one or two times, I know it. Learn them as you need them, not all at one time, you'll never memorize them unless you're actually doing them repetitively. And trying to make sense of modifier keys will just frustrate you.