1. This is the most delicious cake I have ever tasted. (This is the most.... = I have never + Vpp + such a + ....)
-> I have never tasted such a delicous cake 
or: I have never tasted any cake as delicous as this one.
2. There are four rooms in house.
-> My house has four rooms.
3. They bought this house ten years ago. (S + Vpp + O + time + AGO = S + have/has + Vpp + FOR + time)
-> They have bought this house for 10 years.

4. I last saw John when I was in Ho Chi Minh City.
-> I have not seen John since I was in Ho Chi Minh City.
5. English is easy to learn.
->It is easy to learn English
6. It took us three hours to open the door.
-> We spent 3 hours opening the door
7. I haven't seen that man here before.
-> It's the first time I have seen that man
8. Mai has three books in her bag.
-> There're three books in Mai's bag
9. She started working as a secretary five years ago.
-> She has worked as a secretary for five years
10. How old are you?
-> What's your age?
11. I've never met such a famous person before.
-> It's the first time I have met a famous person
12. What is the price of this book?
-> How much is this book?
13. Getting to the art- gallery is easy.
-> It is easy to get to the art- gallery
14. How about watching TV tonight?
-> Why don't we watch TV tonight?
15. It is interesting to learn English.
-> Learning English is interesting
16. How long is this river?
-> What's the lenght of this river?
17. Where do you live?
→ What's your address?
18. I go to school by bike
-> I use my bike to go to school