Question

i need to redirect all posts from the old wordpress to a subdomain

ex:

old-domain/category/photos should redirect to new-domain/category/photos

but if old-domain/ only this should not redirect to the new url

Answer 1

Try adding the following to your htaccess file in the root folder of old-domain

RewriteEngine on
RewriteBase /

RewriteCond %{HTTP_HOST} ^old-domain\.com$ [NC]
RewriteCond %{REQUEST_URI} !^/$ [NC]
# not assets e.g images
RewriteCond %{REQUEST_URI} !\.(css|png|gif|jpe?g|js)$ [NC]
RewriteRule (.+) http://new-domain/$1 [L,R=301]

Edit: modified rule so everything except the home page is redirected