=>these exercises are done quickly by Nga

2. they have to pay the bill before the first of the month.

=> the bill has to be paid before the first of the month.

3. they will ask you some questions.

=>You will be asked some questions.

4. Somebody has locked the box and I can't open it.

=>the box has been locked  and I can't open it.

5. I am doing this work now.

=>This work is being done by me now

6. the boys has painted this wall for 2 days.

=>This wall has been painted by the boys for 2 days.

7. Someone will collect the paper from the room.

=>the paper will be collected from the room.

8. the students were keeping the food in the boxes when we came.

=>the food was being kept in the boxes when we came.