Why the hpprime error messages are not explicit?

[Joe Horn]

(05-26-2016 06:41 PM)compsystems Wrote:  part(-x,2) "Error: Bad Argument Value"
error in the first or second argument?

Both. You're asking for the second part of an expression that contains only one part. Try either part(-x-y,2) or part(-x,1). The error messages are not intended to teach the user how to use the functions. That's what the documentation and Help system are for.