return of extra money on Beijing subway

1) When you buy the card and top up money on the card, what would happen if you do not use that money on each day? Because I heard that your ticket is for one day. I am going to be 4 days there, can I buy the card for all my trip and if I do not use all my money, do I get it back the extra money which I have not used?
2) Does my ticket from the Airport cover my transfer to subways, or I need to get separate ticket for my internal (subway) trips?
Do I need to get a ticket each time I change the subway line?
Thanks.