pop-up that asks a question

I would like to create pop-ups that respond to options that are chosen. For
example, I have a form with a drop down for yes/no. If the answer is yes I
want to be able give the user as option to chose whether radiation was
completed or not. I know how to connect the main form to a subform and have
the subform pop up, but I don't want the whole subform to pop-up -- only one
option.

Thanks