Hello Michael, let's be sincere, the hp-50g really works in RPN, In RPN you have to have the argument in the stack or command line before a function or command is executed and if you want to enter an expression or equation in RPN you have to enclosed by 'single quotation marks' the key associated with the letter O. Now if you want to define a function, let's say AREA(L,W)=L*W, you do this:
['] (ALG appears in the right upper corner of the screen)
then enter AREA(L,W)=L*W
you can now press [ENTER] and then [left-shift][DEF]
['] (ALG appears in the right upper corner of the screen)
then enter AREA(L,W)=L*W
you can now press [ENTER] and then [left-shift][DEF]