1. Choose the correct variant: 1) You ___ (must / can) drive carefully near the crossing. 2) The water ___ (was able to / couldn’t) be cold. It has been very hot these days. 3) The elephants ___ (mustn’t / are not able to) stand the mice. 4) My Granny ___ (could / mustn’t) speak several languages. 5) People ___ (mustn’t / are not able to) litter in the city. 6) He was weak and everybody ___ (had to / was able to) beat him. 7) (Can / Could) ___ you hear a strange noise out there? 8) We ___ (could / had to) smell something burning. 9) The children ___ (are able to / mustn’t) talk with their mouthful. 10) She is a perfect psychologist. She ___ (was able to / had to) persuade me to come.

1. You must drive carefully near the crossing.
2. The water couldn’t be cold. It has been very hot these days.
3. The elephants are not able to stand the mice.
4. My Granny could speak several languages.
5. People mustn’t litter in the city.
6. He was weak and everybody was able to beat him.
7. Could you hear a strange noise out there?
8. We could smell something burning.
9. The children mustn’t talk with their mouthful.
10. She is a perfect psychologist. She was able to persuade me to come.

Обоснование: must выражает обязанность, mustn’t — запрет, could — способность или возможность в прошлом, а was able to — успешное выполнение конкретного действия в прошлом.