4. How often does He study in the library?
--> he usually studies in the library 
5. How often does Hoa write to Tim? 
--> Hoa writes to Tim every month 
6. How often do they play tennis? 

--> they play tennis every week
7. How often do the students go camping?
-->  they go camping twice a year
8. How often does Nga's father watch TV?
--> he watches TV every night