SImplenews module. Where does the form come from?
To modify a form in Drupal you dont need to get where the form is built.

You can make use of hook_form_alter to modify the form 


  Perform alterations before a form is rendered. One popular use of this
  hook is to add form elements to the node form.


hook_form_alter(&amp;$form, &amp;$form_state, $form_id);



  Parameters
  
  $form: Nested array of form elements that comprise the form.
  
  $form_state: A keyed array containing the current state of the form.
  The arguments that drupal_get_form() was originally called with are
  available in the array $form_state['build_info']['args'].
  
  $form_id: String representing the name of the form itself. Typically
  this is the name of the function that generated the form.


What is a Hook ?

Basically they provide a way for a module to extend the functionality of another module. They would go in a module and 'hook' would be replaced by the actual module name. 

Please try  

Examples for Developers

Drupal Hooks Explained