1. She suddenly fell down on her way to school; she must have been ill. (一定是生病了)
2. They ought to have prepared the work for the meeting more carefully. (本应该做好准备工作)
3. There was plenty of time; she needn’t have hurried. (本不必这么匆忙)
4. I could have gone there but my sister was ill. I had to stay at home and take care of her. (本能够去那里)
