htaccess rewrite rules - domain/subdomain https/www - apache

Question

I have my main domain and a subdomain. Subdomain is located in a subdirectory on the root directory of main domain and this cannot be changed. Main domain has an SSL while subdomain doesn't.
Now I am trying to force https and www for the main domain and force no https and no www for the subdomain. So at the end the main domain disregarding how it will try to be accessed should redirect at https://www.example.com and same time subdomain should be http://sub.example.com
All other possible combinations of accessing them should redirect to the above 2. So far, I am successfull to do this for the most possibilities but I am failing to do so for the case when I access the subdomain with https://sub.example.com. It won't redirect to the http version, rather it will try to load the page through https, browser throws the security warning about the SSL and if I proceed, it load the contents of the main domain site through the subdomain and the insecure warning on the browser.
Following is the htaccess rules I am trying, on the htaccess on the root directory, that affects both main site and subdomain.
How to improve them and achieve the desired outcome?
RewriteCond %{HTTPS} on
RewriteCond %{HTTP_HOST} sub\.example\.com [NC]
RewriteRule ^(.*)$ http://sub.example.com/$1 [R=301,L]
RewriteCond %{HTTPS} on
RewriteCond %{HTTP_HOST} sub\. [NC]
RewriteRule ^(.*)$ http://sub.example.com/$1 [R=301,L]
RewriteCond %{HTTP_HOST} !^www\.example\.com [NC]
RewriteCond %{HTTP_HOST} !sub\. [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [R=301,L]
RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} ^example\.com [NC]
RewriteRule ^(.*)$ https://www.example.com/$1 [R=301,L]
RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} ^www\.example\.com [NC]
RewriteRule ^(.*)$ https://www.example.com/$1 [R=301,L]
RewriteCond %{HTTP_HOST} www\.sub\. [NC]
RewriteRule ^(.*)$ http://sub.example.com/$1 [R=301,L]

Answer 1

To redirect only the main domain to https, you can use the following rule :
RewriteEngine on
#https to http (subdomain)
RewriteCond %{HTTPS} on
RewriteCond %{HTTP_HOST} ^sub.example.com$
RewriteRule ^ http://sub.example.com%{REQUEST_URI} [NE,L,R]
#main domain
RewriteCond %{HTTP_HOST} ^example\.com$ [OR]
RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} ^(?:www\.)?(example\.com)$
RewriteRule ^ https://www.%1%{REQUEST_URI} [NE,L,R]