Fatal error: Cannot use object of type stdClass as array in
There are two related but distinct concepts that seem to be conflated in your code, i.e. hook_form, and the notion of a drupal form builder function. Hook_form uses the template explained by @gisleHannemyr.

The template for a form builder function is:

function mymodule_example_form($form, &amp;$form_state) {  
  ...
  return $form;
}


Use hook_form to create or override node creation forms, and a form builder function for any other (non-node-related) form.

If you simply want to alter an existing (node or other) form, you should use hook_form_alter, instead.