Answer from cs61c-eb (Dae-Yeon Kim 16568138) for Question 1
Virtual address is can't be used directly to access main memory, because it is not a real address but an address made to help programmers the burden of figuring out the physical address.  On the other hand, physical address is used to access the main memory directly.
Virtual page number (front 20 bits invirtual address) is used as an index in the page table that contains physical page number, and that physical page is used as a way to get to the physical address.