9. The book is very interesting. I lent it to you yesterday.
→ The book which/that I lent you yesterday is very interesting.

10. Teacher's Day is a day. All the students show their gratitude to their teacher on that day.
→ Teacher's Day is the day when all the students show their gratitude to their teacher.

11. The man and the elephants are from Africa. You saw them on TV last night.
→ The man and the elephants that you saw on TV last night are from Africa.

9. The book that I lent to you yesterday is very interesting.

10. Teacher's Day is a day when all the students show their gratitude to their teacher.

11. The man and the elephants that you saw on TV last night are from Africa.