13. The next train to Bristol **starts** at 8:45 a.m., so I must go now.
14. In recent decades, people's health **has been threatened** by many diseases caused by pollution.
15. If you **were** the president, what would you do to help the environment?
16. If you want to reduce the light, I **will turn off** some LED bulbs.
17. I **have never witnessed** any natural disaster in my life.
18. The football match **will end** at 9 p.m tonight.
19. The concert **begins** at 7:30 p.m. and **lasts** two and a half hours. Don't be late.
20. I **was doing** my homework at 8 p.m. yesterday.