How to get work the hook_form?
hook_form() is only meant to be used by a module that provide content type:

This hook, implemented by node modules, is called to retrieve the form that is displayed to create or edit a node. This form is displayed at path node/add/[node type] or node/[node ID]/edit.

Emphasis mine.
It provides the form, and does not do alterations.
If you want to modify base form created by another module, you need hook_form_alter():

Perform alterations before a form is rendered.
One popular use of this hook is to add form elements to the node form. When altering a node form, the node object can be accessed at $form['#node'].

Again, emphasis mine.