50. We mustn’t break the    (制度）in our school.

49. Every student should wear    (校服）on Monday.

48. My mother often tells me not to (打架)with others.

47. We must _ (遵守）the school rules.

46. She usually_ (到达）at school early.

45. I have to practice _ (play) the piano every day.

44. Remember_  (close)  the windows when  you leave the classroom.

43. My mother doesn’t let me    (read) in bed.

42. She must__(finish) her homework first.

41. _ (not talk). It’s time for   class.