In your query you can use and expresson like this
TermsCode: IIf([Forms]![MyForm]![TheCheckBoxOnTheForm]=Yes,"Prepaid",[Terms])
Where
MyForm is the name of your form
TheCheckBoxOnTheForm is the name of the check box on the form
[Terms] is the name of the query field that contains the terms value you want displayed if the checkbox is unchecked.
OM Gang