Sure! Here are the sentences rewritten while maintaining their original meanings:

27. He said that he had received the envelope.

28. They stated that they visited Cannery Row.

29. She mentioned that she felt very sick.

30. He said that he had bought some popcorn.

31. They announced that they were flying to California.

32. He stated that he was cutting off the main valve.

33. Nam mentioned that his uncle often plants vegetables in the afternoon.

34. She said that her uncle would not plant the vegetables tomorrow afternoon.

35. Nga said that Mr. and Mrs. Pike had just read those newspapers.

36. She mentioned that she often listens to music at night.

37. Lan’s mother said that her daughter usually brushes her teeth before meals.

38. They said that they had to do the assignments now.

39. Lan said that his sister seldom rides a bike to school.

40. They remarked that their teacher sometimes explains the difficult lesson.