1. They decided to go for a picnic though the weather was very cold.

=> Despite the cold weather, they decided to go for a picnic.

2. I have never read this book before.

=> This is the first time I have never read this book.

3. They haven't finished their projects yet.

=> Their projects haven't been finished by them yet.

4. He was very tired. He went to bed early.

=> He was so tired so that he went to bed early.