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.