You almost got it:
Options +FollowSymLinks -MultiViews
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^szop\.in$ [NC]
RewriteCond %{REQUEST_URI} !^/admin [NC]
RewriteRule ^ http://subdomain.szop.in%{REQUEST_URI} [R=301,L]
You cannot use the negation on the RewriteRule
like that, you use it on a conditions.
This should do what you want, it verify if domain is szop.in
and if folder is not /admin
and redirect to subdomain.szop.in
.