Sure, here are the transformed sentences:

27. Nobody can answer his questions. → His questions can't be answered by anybody.
28. My parents made me change my mind but I didn't. → I was made to change my mind by my parents but I didn't.
29. She made me tell the truth. → I was made to tell the truth by her.
30. Jack saw the car drive up outside the police station. → The car was seen driving up outside the police station by Jack.
31. We overheard them say that they didn't really like the food. → They were overheard saying that they didn't really like the food.
32. Jane has just begun learning English for 2 weeks. → English has just been begun to be learned by Jane for 2 weeks.
33. People continue to use Facebook as a main way of socializing. → Facebook continues to be used as a main way of socializing.
34. You need to write a thank-you letter when you come home. → A thank-you letter needs to be written when you come home.
35. The chief engineer was instructing all the workers of the plant. → All the workers of the plant were being instructed by the chief engineer.
36. Will they hold the ceremony at the square next week? → Will the ceremony be held at the square next week?
37. They were doing a physical experiment when I was there. → A physical experiment was being done when I was there.
38. Nobody told me that they had discharged John from hospital. → I wasn't told that John had been discharged from the hospital.
39. They had taken the victim to hospital before the police came. → The victim had been taken to the hospital before the police came.
40. By the time you come back, they will have finished the project. → By the time you come back, the project will have been finished.