1)David lives on/at/in a big city now. He moved from the countryside two years ago.  2)Our garden is full with/of/in flowers in springtime
3) You may/couldn't/should look for a house near public transport. (I advise you to)
4) Sam says he may/had to/could move to the countryside. (It's possible)
5) Guests are required to show their passports at reception. Guests must/have to/should