Baclink

Im new in SEO...

It is necessary to build more baclinks to sub-pages other than the main page?

How does it affects the site rankings?

You have to focus mainly on Main page , but also do seo for inner pages it affects much in google .

The way I see it is that each page should be optimized and backlinked separately. Not just the one page you want to focus on. If your pages are deep linked to each other, optimizing one may bring traffic to your site and lead to the page you originally optimized and backlinked to the most.

For each page you should have individual and corresponding title and keywords. It is better to built links for the main and inner pages targeting the corresponding keywords. Based on the keywords search, your page will be ranking.

Hogward said it accurately. Each of your pages is seo'd for particular kw's and each is backlinked for those kw's.