Ответ:

5. Mathematics is more difficult than History.

6. She is the prettiest girl in her class.

7. Tim is happier than Harry.

8. Tom is more careful than John.

9. Dan is the worst student in the class.

10. Karen is more beautiful than Nora.

Сравнительная степень прилагательных обычно образуется с помощью -er или more и союза than; превосходная — с помощью -est или most. Исключение: bad — worse — worst.