12. The old man was warned about/of the danger.

11. He devoted himself to improve/ improving our environment.

10. The man stood / standing in the front used to be a doctor.

9. Only you can/can you go there to join in the games.

8. Only in this way can we/we can learn English well.

7. The little boy was observed playing /play near the river at this time yesterday.

6. It is worthwhile /worthy to visit the place of interest.

5. His health has been in bad state/condition since he was a young boy.

4.　 The policemen observed the thief entering/enter that building and decided to surround it.

3.　　　 Careless driving cost/took him his life.