I am new at trying to edit the htaccess file.

This situation is pretty confusing to me and I hope I can state the problem in a clear way.

I have a client who has a page that needs to be redirected to another domain, so I added redirect 301 index.htm http://www.newdomain.com/ to the htaccess file. Worked like a charm, however...

The problem is that he has a sub domain on the first domain and every call to that second domain went to the newdomain.com

Does anybody have a solution? I have spent over an hour searching through the various forums here and have not found an answer. Thanks in advance.

The problem is that he has a sub domain on the first domain and every call to that second domain went to the newdomain.com

As it would! .htaccess affects the physical folder architecture of the website NOT the virtual structure.
So a .htaccess file in the root folder (public_html ) also applies the directives to all subfolders, and the default configuration for cPanel or Plesk creates subdomains as subfolders of public_html

Two possible solutions;

Remove and recreate the subdomain in a folder in the user folder so it is at the same level as public_html.

Make the redirect conditional to exclude requests for the subdomain.