"You must not do this" implies that something should not be done because of bad consequences. "You shall not do this" implies that something should not be done because of the speaker's authority. There is some overlap but still a distinctive difference in meaning.

The KJV bible's commandment section is full of "thou shalt" and "thou shalt not" because it does not convey rules but commandments. While a modern reading of a lot of kosher/trefe distinctions and commandments conveys hygienic and health reasons, the science of the day was not sufficient for formulating a hard cause/consequence relationship. The commandments answer the "but why?" question essentially with "because I say so". Hence the use of "shalt/shalt not".

When the subject is first person ("I"/"we"), the difference is more marked: "I shall do this" strongly conveys a personal intent to go through with some act while "I must do this" conveys feeling compelled to go through with it.

"You must not do this" implies that something should not be done because of bad consequences. "You shall not do this" implies that something should not be done because of the speaker's authority. There is some overlap but still a distinctive difference in meaning.

The KJV bible's commandment section is full of "thou shalt" and "thou shalt not" because it does not convey rules but commandments. While a modern reading of a lot of kosher/non-kosher distinctions and commandments conveys hygienic and health reasons, the science of the day was not sufficient for formulating a hard cause/consequence relationship. The commandments answer the "but why?" question essentially with "because I say so". Hence the use of "shalt/shalt not".

When the subject is first person ("I"/"we"), the difference is more marked: "I shall do this" strongly conveys a personal intent to go through with some act while "I must do this" conveys feeling compelled to go through with it.