Ответ:
1. George has travelled a lot. He can speak four languages.
2. I haven't been able to sleep very well recently.
3. Sarah can drive but she hasn't got a car.
4. Sorry, I can't see you on Friday, but we may be able to meet some time next week.
5. My grandfather was a very clever man. He could speak five languages.
6. I can't understand Martin. I have never been able to understand him.
7. I used to be able to stand on my head but I can't do that anymore.
8. Ask Catherine about your problem. She might be able to help you.
9. She spoke in a very low tone, but l could understand what she said.
10. Laura hurt her leg, so she couldn't walk very well.
11. I'm afraid I can't come to your party next weekend. I have to help Marvin paint his room.
12. We tried hard but we couldn't persuade him to come with us.