1. You sent me a present. Thank you very much for it.

Thank you for the present (which/that) you sent.

2. Romeo and Juliet were lovers. Their parents hated each other.

Romeo and Juliet were lovers whose parents hated each other.

3. This is Mrs Jones. His son won the championship last year.

This is Mrs Jones, whose son won the championship last year.

4. I was sitting in a chair. It suddenly collapsed.

The chair (which/that) I was sitting on suddenly collapsed.

The chair on which I was sitting suddenly collapsed.

5. His girlfriend turned out to be an enemy spy. He trusted her absolutely.

His girlfriend, whom he trusted absolutely, turned out to be an enemy spy.

6. This is the story of a man. His wife suddenly loses her memory.

This is the story of a man whose wife suddenly loses her memory.

7. The firm is sending me to London. I work for this firm.

The firm ( which/that) I work for is sending me to London. 

8. The horse kept stopping. This annoyed the rider.

The horse kept stopping, which annoyed the rider.

9. Tom had been driving all day. He was tired and wanted to stop.

Tom, who had been driving all day, was tired and wanted to stop.

10. This is the story of a group of boys. Their plane crashed on an uninhabited island.

This is the story of a group of boys, whose plane crashed on an uninhabited island.