How can I force a field to be required if another field has a certain value?
With custom code you can do something like this:

add a new validation function using hook_form_alter()

your_module_form_alter($form, $form_state, $form_id) {
  if($form_id == 'the_form_ID') {
    $form['#validate'][] = 'your_custom_validation_function';
  }
}


then check to see if the conditions are met

function your_custom_validation_function($form, &amp;$form_state){
  if( $form_state['values']['your_checkbox_name'] == 1 ) {
    // check to see if tags are set
    // use form_set_error('tags_field_id', 'error message') to set a error message
  }
}