Como fica o verbo "Mustn't ou Must not" no Reported Speech?

Na frase "Roberto says: I mustn't open the door.", no Reported Speech, dizem que o "Must" vira "Had to" no passado, então, se essa frase fosse para o Reported Speech, ela ficaria "Roberto said that he didn't have to open the door." ? Tenho essa dúvida porque, se o "Mustn't" implica uma proibição, o "Didn't have to" não seria algo do tipo "É permitido abrir a porta, porém não é necessário" ? Fica meio contraditório essa parte, se alguém pudesse me explicar melhor eu ficaria muito grato.

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2 respostas
Henry Cunha 3 18 191
You're right that "must not" implies a prohibition, or a strong aversion to doing something, while "don't have to" implies having no need to, or not being obliged to do something.

You don't necessarily need to switch from the present tense for "must":
"I mustn't open the door," says Roberto.
Roberto / is saying /says / said / he mustn't open the door.

But if you really wish to, we probably need to know what "must" means in this case. It could be because of a personal decision not to, or because of some rule by somebody else, as in the following:

Roberto said he wouldn't / couldn't open the door.
Roberto said he was not to open the door.
Etc.
Henry Cunha 3 18 191
In other words:

"I must" means the same thing as "I have to".

But "I must not" does not mean "I don't have to".

Sometimes we can use "must" invariably as to time:

He must have been saying one thing and I understood something else.
Option: He had to be saying...

But the status of the situation may lead us to change from "must" to something else:

Door not yet open:
He said the door must be opened.
He said the door must not be opened.

Door already opened:
He said the door had to be opened.
He said the door was not to be opened.
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