Hi Wim,
do you want to auto-fill fieldA depending on fieldb? If yes, look at my example:
I use two fields, fieldB is the Salutation, fieldA is the SalutationString which should be used as title in a Form Letter. SalutationString should be auto-filled with "Dear Mr." if Salutation contains "Mr.", with "Dear Ms." if Salutation is "Ms." and with "Dear Sir or Madam" if Salutation is blank.
To do this, open the Field Definition dialog, click on SalutationString (in your case fieldA)and open the Option Dialog. In the Option Dialog box DefaultValue check the Modification Formula Radio Button and then press the Formula Button.
Now you can enter the Formula:
If(Salutation = 'Mr.'; 'Dear Mr.'; If(Salutation = 'Ms.'; 'Dear Ms.';'Dear Sir or Madame'))
You can do that manually or by using the Formula Assistent. The assistant is helpful, because you will see the syntax of the choosen funktion in the bottom line Formula dialog.
Hope it works,
Lu