Overriding template without killing the page? I would like to override the template for node creation. In the past when I've overridden a template, I've usually just copied the theme's generic template for that type (page, node, etc), given it the proper name, and edited it as I saw fit.

Now, for the first time I need to override a template that I can't simply make a copy of. I'm editing the node editing template and I have created a file with the correct name and location and everything, and it overrides the edit form, all right. Now there's nothing on the page except what's in my new template (unsurprisingly). 

So, how do I create a template without getting rid of what was already there? Or, where can I find the template that Drupal is using to create my node edit form?

I'm using Drupal 7 and the theme is a custom one written by the previous maintainer of the site. It's not a child theme of a base theme, it's all by its lonesome.
Because the Node Edit page is a form, it is recommended that you use hooks for theming it.  You can create custom template file for it, but theming it within template.php is the most common solution.

Theming Node Forms in Drupal 7 is the best resource I've seen so far for this.