Here are the completed sentences based on your prompts:

1. With the efficient public transportation system, **traveling around the city becomes much easier.**

2. The apartment is big. The rent is high.
**The bigger the apartment, the higher the rent.**

3. As he gets older, he wants to travel less.
**The older he gets, the less he wants to travel.**

4. Please tell me how I can get to the bus station.
**Please give me directions to the bus station.**

5. She trains harder; she can run faster than you.
**If she trains harder, she can run faster than you.**

6. I'm not sure what I should cook for my son's birthday party.
**I'm not certain about the menu for my son's birthday party.**

7. My son made the decision to spend less time playing games. (CUT)
**My son decided to cut back on his gaming time.**

8. Do you have good relationships with your classmates? (GET ON WITH)
**Do you get on well with your classmates?**

9. I don't have a computer.
**I wish I had a computer.**