11. China has the largest population in the world.

=> There are large population in China

12. The book really interests me.

=>  I like the book because it is very interested 

13. They didn’t go fishing because it was too cold.

=>  If it hadn't been too cold, they would have gone fishing

14. When did you start learning English ?

=>  How long have you learnt English?

15 . I’m sorry I was so rude to you yesterday.

=> I apologized to Tom for having been rude to him the day before.

16. People say she is an honest and reliable person

=> it is said that she is an honest and reliable person

17. It took me two hours to do my homework.

=> I doing mu homework 2 hours ago 

18. He left school and then joined the army.

=>  After having left school, he joined the army.