Model: The man gave her the book.
-> It was the man who gave her the book.
1. Mr. Smith teaches us English.
=> it is mr. Smith who teaches us english
2. The bos are playing in the garden.
=> it is the garden in which the bos are playing
3. I'll go to Ho Chi Minh city tomorrow.
=> it is tomorrow when i 'll go to ho chi minh city
4. We haven't eaten this kind of food before?
=> this is the kind of food we haven't eaten before
5. A lot of tea is drunk in China.
=> it is chhina in where a lot of tea is drunk