1. The children really need a garden in which they can play=> The children really need a garden to play in
2. Your son was the second child who was kidnapped in this way=>Your son was the second child to be kidnapped in this way
3. There is a swimming pool in which we can swim=>There is a swimming pool to swim in
4. We visited Hanoi, which is the capital of Vietnam=>We visited Hanoi, the capital of Vietnam.
5. My father, who is a pilot, often goes abroad=> My father, a pilot, often goes abroad