Why cannot I print $form in my template file? I have a "video" content type. I want to customize the template file for edit form,
page--node--add--video.tpl.php, but when I render (or print) $form, it returns NULL.

What is the problem?
There's no $form variable available in page.tpl.php (see the docs for a full list of available vars).

If you're trying to output the page content it's done like so:

&lt;?php echo render($page['content']); ?&gt;