Here are the transformed sentences based on the provided prompts:

1. **The hotel in which we stayed during summer vacation was luxurious.**

2. **The day when they met again was a beautiful day.**

3. **The jokes that Michel laughed at when I told him were funny.**

4. **The first man who walked on the moon was Neil Armstrong.**

5. **These shoes are too small for my feet.**

6. **The tea was not cool enough for her to drink.**

7. **The police let people enter the exhibition.**

8. **Although Mary felt depressed, she went to her friend's birthday party.**

9. **Henry's wallet was empty, with nothing inside.**

10. **If you are patient, you will succeed in life.**

11. **My grandfather’s 80th birthday will be celebrated by my family next Saturday, on May 25th.**

12. **We will invite many friends to join the picnic; our friends are our classmates.**

13. **We feel sorry that he can't join the dinner at my house tonight.**

14. **If you come to our house at about 5:00 PM, you can meet all our old friends.**

15. **Though it's too late for bedtime, her son hasn't arrived home yet.**