IV.
1. This is the house in which we often stay in summer.
2. I'll show you the second hand bookshop in which you can find valuable books.
3. I have not decided the day on which I'll go to London.
4. He was born on the day on which his father was away.
5. The book in which we can look up telephone numbers is a telephone directory.

IV.
6. Mrs. Brown rang Dr. Brown who was going to carry out an urgent operation at that moment.
7. The airport where we're going to arrive is the most modern one.
8. She doesn't want to speak of the cause why she divorced her husband.
9. The days when I lived far from home were the saddest ones.
10. I never forget the park where we met each other for the first time.

IV.
1. This is the house in which we often stay in summer.
2. I'll show you the second hand bookshop in which you can find valuable books.
3. I have not decided the day on which I'll go to London.
4. He was born on the day on which his father was away.
5. The book in which we can look up telephone numbers is a telephone directory.