1. I don't know the answer so I can't tell you.
→ If I knew the answer, I would tell you.
2. The media are going to attack the famous people.
→ The famous people are going to be attacked by the media.
3. She didn't know you were in hospital, so she didn't visit you.
→ If she had known you were in hospital, she would have visited you.
4. Mary phoned me. She wanted to invite me to the party.
→ Mary phoned me in order to invite me to the party.