1. You didn't tell us the reason. We have to cut down our expenses for that reason.
=> You didn’t tell us the reason for which we have to cut down our expenses.
2. I don't know the girl's name. She just gone into the hall.
3. Jack's car had broken down. He had to take a bus.
- Jack,whose car had broken down, had to take a bus.
5. I spoke to a shop assistant. She was very young
- the shop assistant who I spoke to was very young.
6. This is the bank. We borrowed money from it.
- This is the bank where we borrowed money.
7. A shop sells these things. It's in Oxford street.> The shop where sells these things is in Oxford street.
8. My step mother is not very kind to me. I'm living with her
- My step mother who I’m living with is not very kind to me.
9. This is Mrs. Jones. Her son won the championship last year
- This is Mrs. Jones whose son won the championship last year.
10. Rod Lee has won an Oscar. I know his sister
- Rod Lee, whose sister I know, has won an Oscar.