41. It is essential for these people not to make mistakes.

　　42. It is important to know your readers' opinions.

　　43. It lets sunlight in and keep much of warmth from getting out.

　　44. It was black in colour, with a head in the shape of a triangle.

　　45. It was so interesting that I forgot the time.

　　46. It was too big for them to take with them.

　　47. It's three meters tall and weighs 207 kilograms.

　　48. It's time to do your maths homework.

　　49. It's very kind of you to help me.

　　50. Just run as fast as you can!