"must" in reported speech

[wynnmyintuu]

How shall I change the auxiliary "must" when changing direct speech into reported speech? What's the difference between "must" kept unchanged and "must" that has to be changed into "had to"?

 

[Raymott]

How shall I change the auxiliary "must" when changing direct speech into reported speech? What's the difference between "must" kept unchanged and "must" that has to be changed into "had to"?

You don't have to change "must" for reported speech.
If "must" means "have to" in the original, then you can change "must" to "had to" when reporting it.

Spoken: "I must work tomorrow"
Reported: "He said he had to [or must] work the next day."

Spoken: "The government must act on this!"
Reported: "He said that the government must act on this."