Well, I’d like to know the relationship of meaning and usages of the words “should”, “must” and “shall” concerning to the assumptions, obligation and advices.
must is stronger than should . Must is used for giving official instructions, or main rules of something.
Form example; The instruments in NYC says drivers must keep their eyes open …
otherwise, you can use both should and must .I realize native speakers use should more than must …
shall is kind of will although it has a slightly difference, you say I am going to go to Boston -similar to- I shall go to Boston.
people tend to not use it anymore.
I hope it is helpful
I suggest you had better wait for the experts and the couches to explain this because I am not pretty sure .
Thanks for your help!