Here are the sentences completed based on the prompts:

1) It started to snow at 5 o’clock.
It has **snowed** since 5 o’clock.

2) When did you go to bed yesterday?
What **time** did you go to bed yesterday?

3) What time does Paul have to leave?
Do you know what **time** he has to leave?

4) This book is very boring, and so is that one.
This book is **as boring as** that one.

5) I last read a book on September 20th.
I haven’t **read a book since** September 20th.

6) We couldn’t answer those two difficult questions.
Those two **difficult questions** couldn’t be answered.

7) We couldn’t drive because of the fog.
The fog prevented **us from driving**.

8) I stopped. I bought a newspaper.
I stopped **to buy** a newspaper.

9) We watched them leave. I will never forget it.
I will **never forget watching** them leave.

10) It took us two hours to water the flowers.
We spent **two hours watering** the flowers.

11) He is extremely rich but he is not happy.
In spite of **his wealth**, he is not happy.