Here are the completed sentences without changing their meanings:

1. That electric car's so expensive that I don't think I can buy it.
→ It’s such a pricey electric car that I don’t think I can buy it.

2. I like living in a smart city more than living in the countryside.
→ I prefer living in a smart city to living in the countryside.

3. This is the first time I have seen this kind of intelligent robots.
→ I haven’t seen this kind of intelligent robots before.

4. Although the air in the city is polluted, he still wants to become a city dweller.
→ In spite of the air in the city being polluted, he still wants to become a city dweller.

5. The police let him leave after they had questioned him.
→ The police allowed him to leave after questioning him.

6. The room in the skyscraper was so expensive that I didn’t buy it.
→ The room in the skyscraper was too expensive for me to buy.

7. It took us only ten minutes to finish the homework.
→ We spent only ten minutes finishing the homework.

8. He has installed solar panels on the roof for three years.
→ He started installing solar panels on the roof three years ago.

9. It was such a dirty beach that I decided not to stay.
→ The beach was so dirty that I decided not to stay.

10. He hasn’t played basketball for 6 months.
→ The last time he played basketball was 6 months ago.