Designers often choose between applying rules at the time individual objects are filled versus applying rules to all objects when a form is submitted or saved. The following describes the first option.
Radio buttons are nice option instead of check boxes. They can be made to look like check boxes and you don't have to manage the alternative response (un-check).
Create a radio button list, called 'answer', defined as an exclusion group with the radio buttons 'yes' and 'no'. Define the item bindings on 'answer' as 1 - yes and 0 - no. Additionally, create a text field called 'comments'.
Given the form root is called 'form1' and a subform called 'subform1', the following events could be added to help enforce comments as a required field.