Ok, I'll tell you my secret how to become an expert in C2
When you are adding/editing an action, there should be a semitransparent window somewhere on the screen - "Object with expressions". If you don't see it, press Ctrl-F4.
Explore everything in this window, it lists all expressions for all objects. The most useful one is System, but others also contain lots of interesting stuff.
That's it That's my secret - I know where to lookup expressions. And now you are an expert too!