Sub-Domain and Login Trouble

Got a domain name set up with the first of several sub-domains associated with it.

The address that works to get to the WP login page is:
https://www.domainname.com/subdomain/wp-admin/ WP is actually installed in the subdomain folder.
Login, and all is well except that when you try and View The Site from here all I get is black text on a white screen. Looks as if the style sheet is missing.

If I use this url to login to WP….https://www.subdomain.domain.com/wp-login.php….the WP login page is pulled up but the login/pw is not accepted.

And lastly, when I pull up the site at subdomain.domain.com, it pulls up fine.

If this makes “sense” to anyone, please tell me why I can’t use the subdomain.domain.com to log in to WP and/or why I can’t get the proper look of the site when I try to view it from the admin area.

And give it to me on a third grade level, please. ??