Sure, here are the completed sentences:

1. What was your favourite subject at school?
=> What subject did you enjoy most at school?

2. If you don't work hard, you will not pass the exam.
=> Unless you work hard, you will not pass the exam.

3. Let's stay at home during the Covid time.
=> What about staying at home during the Covid time?

4. I think that no city is more beautiful than Paris.
=> I think Paris is the most beautiful city in the world.

5. The computer was too expensive for me to buy.
=> I didn't have enough money to buy the computer.