4. He desires he can fly which is unrealistic. 
5. Lady Gaga, who is a well-known pop star, is only 24.
6. He works with his parents whose company makes furniture. 
7. She ordered me a coffee which was just what I needed.
8. Tam Anh Hospital, which local people depend on, is being closed down.
9. My best friend Tom, who is near my house, gave me an early birthday present.  
10. Tom's mother, who lives in Poland, has 4 grandchildren.
11. My grandfather, who came from Viet Nam, is dead now.
12. They stopped at the museum which they had never visited before.
13. They have 2 daughters who are reporters.
14. Mount Everest, which is a part of the Himalayan range, is the highest mountain in the world.
15. My mother, who is a vegetarian, never eats meat.
16. Titi, who is my lovely cat, likes to eat fish.
17. There is a cat whose leg is broken. 
18. My high school, where I study, is one of the greatest schools in town.  
19. July, when my birthday is celebrated, is my favorite.
20. East High School, which we used to go to that school, has been closed down.
``
Thay thế cho 
- người: who
- vật: which
- sở hữu cách: whose
- địa điểm: where
- thời gian: when


Mệnh đề quan hệ:
- xác định:
+ không có phẩy
+ mệnh đề phía sau đại từ quan hệ không thể lược bỏ
+ có chủ ngữ chưa xác định
- không xác định:
+ có dấu phẩy
+ mệnh đề phía sau đại từ quan hệ có thể lược bỏ
+ có chủ ngữ đã xác định (Tên người, địa điểm nổi tiếng, sở hữu,...)