Sure! Here are the completed sentences:

31. Have you ever climbed a mountain before?
→ Is this **your first time?**

32. I started reading the book last night.
→ I have **started it.**

35. This is the first time I have eaten out at a 5-star restaurant.
→ I haven't **eaten out at a 5-star restaurant before.**

36. I haven't met my best friend since she went to study abroad.
→ I last **met her before she went abroad.**

37. It is two years since we last went on a picnic together.
→ We haven't **gone on a picnic together in two years.**

39. I last mopped the floor 3 days ago.
→ I haven't **mopped the floor since three days ago.**

61. They have never had such a cold winter.
→ It is the **coldest winter they have ever had.**

72. I’ve never read a more romantic story.
→ It's the **most romantic story I've ever read.**

73. This is the best computer I have ever used.
→ I have never **used a better computer.**

74. That's the worst film I've ever seen.
→ I've never **seen a worse film.**

2. We only came by bus because there were no taxis.
→ If there **had been taxis, we would have come by taxi.**

4. He smokes too much so he can't get rid of his cough.
→ Unless he **quits smoking, he can't get rid of his cough.**

11. If you don't start at once, you'll be late.
→ Unless **you start at once, you'll be late.**

14. Go to Britain and you'll be able to speak English all day long.
→ If **you go to Britain, you'll be able to speak English all day long.**

16. Write to her, or she will forget you.
→ If **you don't write to her, she will forget you.**