Must is another modal verb and will behave as such, meaning that all the peculiarities regarding modal verbs will apply to this one as well.
MUST-OUGHT TO have three type of meanings and specific usage accordingly:
(a) indicating obligation, an order given by the speaker, as for instance:
You must do your homework before dinner
However, there are subtle differences in meanings that may require other verbs such as Need/Have to both less formal than the previous one they also imply slight differences, for example if one says:
You need to (informal have to) have a valid driver’s license to drive (it implies that the document is a requirement by law)
Whereas the following statement implies that the speaker feels obliged:
I need to (informal have to) go now (meaning that I feel the obligation to leave for some reason, although it’s not necessarily my wish to do so, in other words something or someone else is controlling the event of my leaving)
Ought to/Should specifically mean that the speaker is not altogether certain that the order will be actually carried out:
You have a terrible cold, you ought to (should) have a cup of warm tea and go straight to bed
(b) indicating advise or recommendation, the speaker advises or recommends an action or behavior, as for instance:
You must study for your exams if you want to pass them
You must (informal have to) read this book!
You need to (informal have to) take care of that tooth ache!
(c) indicating the drawing of a conclusion, the speaker has the knowledge to draw a onclusion regarding something, as for instance:
You must be tired after so much studying
They look exactly alike, they must be twins (there’s no doubt to this obvious conclusion)
He ought to (should) make it back in time for dinner (there is less confidence in this statement but fairly enough to believe that the person will in fact be in time)