Hi,
What you are saying is absolutly correct that keywords as well as reserved words we should not use it for identifiers.
To me I could differentiate only one thing. The reserved words 'true', 'false' and 'null' are values. I mean like numbers, alphabets for human's these are values for computers.
I guess so... don't worry much about this. But
you should know which is keyword and which one is reserved words. That's all needed for the exam.
thanks,
Surya K