Теория:

Если вводный глагол стоит в прошедшем времени, модальные глаголы изменяются в косвенной речи следующим образом:

can — could.
Пример:
she said, ‘I can speak four languages.’ — She said (that) she could speak four languages;
maymight.
Пример:
‘I may call you,’ Sue said. — Sue said that she might call us;
shall — should.
Пример:
she said, ‘When shall I call you?’ — She asked when she should call us;
must — must/had to.
Пример:
the teacher said, ‘You must write the report on Friday.’ — The teacher said that I had to / must write the report on Friday;
needn’t — needn’t / didn’t need to / didn’t have to.
Пример:
‘You needn’t go right now,’ Dad said. — Dad said that I needn’t / didn’t need to / didn’t have to go at once.
Глаголы would, could, might, should, mustn’t в косвенной речи не изменяются.