WordPress, non-wp pages and .htaccess

We have an existing site at https://www.site.com. I’ve now moved it to https://www.site.com/sub/newname.php. I have modified .htaccess to make this the default page for the site and that is working

This site will soon be replaced by a WordPress site but I need to get WP up and running in the ROOT directory of this server, while the above address is still the default. I have WP properly installed and if I remove the current .htaccess file I can see it.

What I need is this:
If someone goes to: https://www.site.com
They get: https://www.site.com/sub/newname.php

If someone goes to: https://www.site.com/index.php
They get the actual wordpress site