Hi Quick questions. If I build backlinks to subpages of my site from high authority site like PR6 or 7 - and after a while those subpages PR goes up - will my main page PR will go up automatically?
Only if there are links on subpages pointing to the homepage. If not, homepage PR won't change, because PR is a characteristic of each individual page.
If your subpages have links that directs to home page,then it'll help to increase the PR of the home page.
If your subpages get some PR then homepage will get it automatically, pagerank is calculated for domain, not for seperate pages, so the pagerank for single page will be distributed to homepage with next update.