1. Some people just don't know how to sing. (be able to) -> Some people are not able to sing. (Некоторые люди не умеют петь.)
2. There's a chance that she's in the airport. (could) -> She could be in the airport. (Она могла бы быть в аэропорту.)
3. Do not block the emergency exit. (mustn't) -> You mustn't block the emergency exit. (Нельзя блокировать аварийный выход.)
4. It is dangerous to use mobile phones on the plane. (mustn't) -> You mustn't use mobile phones on the plane. (Нельзя использовать мобильные телефоны в самолете.)
5. It is necessary to fasten your seat belt for landing and taking off. (must) -> You must fasten your seat belt for landing and taking off. (Вы должны пристегнуть ремень безопасности при посадке и взлете.)
6. This is a surprise. Don't tell anybody about it. (shouldn't) -> You shouldn't tell anybody about it. (Не следует никому рассказывать об этом.)