Use CAPTCHA module on a custom HTML form? (no use of Drupal Form API)
I'm wondering maybe my custom HTML form doesn't have a $form_id because it doesn't use any drupal form modules? If that's the case, then I probably can't use the CAPTCHA module. Could someone please confirm?


Correct - the Captcha module has been written exclusively for integration with forms built by the core form API. Without a lot of changes, it's not possible to use any of its functionality on a form that hasn't been built by Drupal.