Sure! Here are the completed sentences to maintain their meanings:

31. She has never climbed a mountain before.
32. This is the first time I have read the book.
33. I started playing basketball one year ago.
34. He began playing basketball one year ago.
35. The children started riding bicycles when they were 5 years old.
36. They have been my best friend since she went to study abroad.
37. I last went on a picnic two years ago.
38. We haven’t had a picnic in two years.
39. This is the first time I have heard her sing.
40. I haven’t heard her sing before.
41. I last swept the floor 2 days ago.
42. I haven’t swept the floor since 2 days ago.
43. The children haven’t been to the amusement park since 2005.
44. The last time they went to the amusement park was in 2005.
45. John last sent me a letter when he was in New York.
46. The last time I heard from John was when he was in New York.
47. She began to write 30 minutes ago.
48. She has been writing for 30 minutes.
49. This is the first time she has eaten eel soup.
50. She has never eaten eel soup before.
51. I last saw him 3 weeks ago.
52. I haven’t seen him since 3 weeks ago.
53. This is the first time I have been abroad.
54. I have never been abroad before.
55. I haven’t gone to the zoo since last year.
56. The last time I went to the zoo was last year.
57. She started to cry two hours ago.
58. She has been crying for two hours.
59. It is a long time since we last went to this restaurant.
60. We have not been to this restaurant in a long time.
61. I haven’t heard from her for many years.
62. The last time I heard from her was many years ago.
63. It’s over six months since John had his hair cut.
64. John hasn’t had his hair cut for over six months.
65. I have never seen a dirtier dog.
66. This is the dirtiest dog I have ever seen.
67. I have never seen such a beautiful building.
68. This is the most beautiful building I have ever seen.
69. They started studying English when they were in grade 3.
70. They have been studying English since they were in grade 3.
71. It is a long time since she last saw her parents.
72. She hasn’t seen her parents in a long time.
73. I haven’t met my aunt since I was 10 years old.
74. I last met my aunt when I was 10 years old.