How to make a checkbox field required when a radio button is selected.
In form alter use this code

function my_module_form_alter(&amp;$form, &amp;$form_state, $form_id) {
      $radio = $form['#node']-&gt;field_radio['und'][0]['value'];

      if($radio == 1){//if 
         $form['field_checkbox']['und']['0']['value']['#required'] = TRUE;
         //OR
         $form['field_checkbox']['und']['0']['#required'] = TRUE;
      }
}