I want to do something simple - but I can't see how FileMaker can do it.

I want to prevent user changes to a field based on the value of another field (in the same record).

Specifically: If FormIssued = "issued" then FormNumber field is read only.

Privilige sets only seem to control editablity of whole records, and validation doesn't kick in until the record commits (which confuses users).

There are probably a bunch of different ways to do this depending on your work flow.

You could have record creation done using global fields with a submit button. After submission, the record becomes non editable in a dispaly where entry to the fields is prohibited.

You could have dual layouts and when going to the screen, if the form is issued, have them directed to the non editable screen.

I am sure that there are others.