I want to know that is it possible that a site's sub page have high PR(Page rank) than its main domain or Home page.
Yes my dear it is possible. If you are creating new page in your website with good content or you are doing seo for that page so definitely you can get higher ranking for that particular page .. and it is not necessary that when you main index page rank will be increased then your inner page rank go higher.. so in a simple way it is possible.
Yes, it is possible. If your inner page receives higher amount of traffic than your homepage..it will rank higher.