I have a 12C (not a Platinum!), and I can not get back to RPN mode

Your HP 12c Platinum is in Algebraic mode. To switch back to RPN mode, press f then CHS.

I take it you mean a 12C Platinum.

Press f ALG. ALG is the f-shifted function of the EEX key, just above the double-size ENTER key. This will switch the 12C Platinum from RPN mode to Algebraic mode, and ALG indicator will light up instead of the RPN indicator.

To switch back to RPN, press f RPN. RPN is the f-shifted function of the CHS key just above the EEX key.

I checked the manual, the image of the face of the calculator and there is no way to change the entry mode from RPN to Algebraic. Your calculator is still in RPN, it is the only mode it has. Reset it.

On a 12C Platinum, press f [ALG]

On a non-Platinum you're stuck. It has only the RPN mode.

On a 12C Platinum, press f [ALG]

On a non-Platinum you're stuck. It has only the RPN mode.

To switch to the algebraic mode press f [ALG] (the EEX key).

Assuming you have a 12C Platinum, press f [RPN] to switch from the algebraic to the RPN mode. [RPN] is the shifted function of the CHS key on the top row of the keyboard.

I presume you're on a Platinum since the non-Platinum 12Cs don't have an algebraic mode.

To switch back to RPN, press f [RPN]. That's the yellow "f" key followed by the "CHS" key.

The HP 12c is a purely RPN calculator; it does not have an algebraic mode.

To switch the HP 12c Platinum to algebraic, press f [ALG].

I assume we're dealing with a 12C Platinum, since a 12C doesn't have a +"equal" sign configuration_.

Press f then RPN.