Validation Problems, Require text box if a radio button is clicked

[djjohnst]

I am creating a form(survey) where the first question is a yes/no
question using two radio buttons. If they click "yes" I want to then
make the next 4 questions required. I am not sure how to do this. I
thought I could use a custom validator, and using code behind use an
IF statement. However, I am not sure how to pass the value from the
radio button group. I am having trouble. If this is easy forgive me, I
am new at this .net game.