Doh! Of course, you are right.

After giving it some thought, I think I can rephrase the problem (taking a different approach) so that it is possible. Maybe.

We have a string (Hablar, for example) in C12. The formula in E12 will check C12 to see if the string ends in "ar", and if it does it writes that string in C12 minus the "ar" ending and appends the "áste" string to it. (If C12 ended in "er" or "ir" it would append the appropriate string instead of "áste".)

A simple formula like

=if(right(c12,2)="ar",C12,"")

is not circular, but when we start getting fancy by writing only part of C12 to the destination cell and then appending something to it, I'm not sure what will happen.

tanstaafl.