2. They ought to have thought of that sooner.
3. Children, would you like me to make something special for dinner tonight?
4. Robert could have forgotten to post the letter.
5. Excuse me, could I have a look at your newspaper?
6. They weren't able to finish the exercise because they didn't have time.
7. Mobile phones must not be used on the plane.
8. Richard can't have stole your camera.
