Miałem napisać coś mądrego ale ostatecznie uznałem, że wstawię fragmenty z książki
"The modals would, should, could, might, ought and must are usually unchanged after past reporting verbs in indirect speech. This is also true of needn't (► 532) and had better (► 77)."
"Must can be used after a past reporting verb as if it were a past tense.
The doctor said that I must stop smoking.
Obligation can also be reported with had to and would have to.
The doctor said that I had to / would have to stop smoking."