1. The student who is from China sits next to me.

2. I thanked the woman who had helped me.

3. The professor whose course I am talking is excellent. 

4. I recently went back to Paris, which is still as beautiful as a pearl.

5. Nam knows the man to whom we talked this morning

6. The house where my grandfather used to live is very old

7. The girl to whom I talked yesterday looked like my mother.

8. The job which I applied for last month is interesting

9. The meeting which I went to yesterday lasted five hours

10. This is my best colleague about whom I told you last time

11. The house which the had lived in for ten years was ruined last month

12. She showed me a photograph of her son, who is a doctor

13. The sea in which we decided not to swim looks rather dirty

14. The new stadium which can hold 9.000 people will be built next month

15. The doctor to whom I went to see last week told me to rest for a few days

16. A friend of mine whose father is the manager of a company helped me get a job

17. We spent a pleasant day by the lake, where we had a picnic

18. The teacher asked us if we understood the question 

19. Next week I'm going to Paris, where my wife works

20. London, which we visited last summer, has a great clock- Big Ben

1/ The student is from China. He sits next to me.

⇒ The student who sits next to me is from China. 

2/ I thanked the woman. This woman had helped me. 

⇒I thanked the woman who had help me. 

3/ The professor is excellent. I am taking his course. 

⇒ The professor whose course I'm talking is excellent. 

4/ I recently went back to Paris. It is still as beautiful as a pearl. 

⇒ I recently went back to Paris which is still as beautiful as a pearl. 

5/ Nam knows the man. We talked to him this morning.
-> Nam knows the man whom we talked to this morning.

6/ The house is very old. My grandfather used to live there. 

⇒The house where my grandfather used to live is very old

7.The girl looked like my mother. I talked to her yesterday.

⇒The girl to whom I talked yesterday looked like my mother.

8.The job is interesting. I applied for it last month.

⇒The job which I applied for last month is interesting

9.The meeting lasted five hours. I went to it yesterday.

⇒The meeting which I went to yesterday lasted five hours

10. This is my best colleague. I told you about him last time.

⇒This is my best colleague who I told you last time.

11.The house was ruined last month. They had live in it for ten years.

⇒The house was ruined last month in which they live for ten years.

12.She showed me a photograph of her son. He is a doctor.

⇒She showed me a photograph of her son who is a doctor.

13.The sea looks rather dirty. We decided not to swim in it.

⇒The sea looks rather dirty which we decided not to swim in.

14.The new stadium will be built next month. It can hold 9.000 people.

⇒The new stadium will be built next month which can hold 9.000 people.

15.The doctor told me to rest for a few days. I went to see him last week.

⇒The doctor told me to rest for a few days when I went to see him last week.

16.A friend of mine helped me get a job. His father is the manager of a company.

⇒A friend of mine helped me get a job whose father is the manager of a company.

17.We spent a pleasant day by the lake. We had a picnic by the lake.

⇒We spent a pleasant day by the lake where we had a picnic.

18.The teacher asked us the question. Do you understand it?

⇒Do you understand the question which the teacher asked us?

19.Next week I’m going to Paris. My wife works in Paris. 

⇒ Next week I'm going to Paris in which my wife works.

20.London has a great clock- Big Ben. We visited London last summer.

⇒Last summer, we visited London where has a great clock- Big Ben.