Here are the completed sentences based on the prompts:

11. How long is it since you last saw Mary?
**I haven’t seen Mary for...** (insert the duration since you last saw her)

12. This is the first time I have visited Hanoi.
**I have never visited Hanoi before.**

13. I began learning English five years ago.
**I have been learning English for five years.**

14. I last saw her in 2010.
**It has been years since I last saw her.**

15. I have never eaten with knives and forks before.
**I usually eat with my hands.**

16. It is 10 years since I last visited Dalat.
**I last visited Dalat 10 years ago.**

17. She hasn't phoned home for two weeks.
**It has been two weeks since she phoned home.**

18. We last went to London two years ago.
**It has been two years since we went to London.**

19. She started driving one month ago.
**She has been driving for one month.**

Feel free to adjust the statements if you need specific durations or other details!