English like most languages are imbued with idiomatic and sarcastic usage.
We know that the following phrases are meant to be courteous polite phrases
- Could you please
- I would
However when used in many contexts, those phrases actually mean to say "Well, I am being as polite and gentle as possible to tell you knuckle-head to do what I think is best."
- Please sit down !
- Could you please shut up ?
- I would not tell my children to walk alone in the night like you did.
Must is a word that connotes mandatory.
- It must be there but it isn't.
- For the fish to survive, you must clean the tank, but you didn't.
Usually, must is used in cohortative subjunctive or imperative subjunctive situations. But what happens when you use the word must in a propositional subjunctive situation?
Reference: Use of subjunctive form
Subjunctive situations have been categorized/quantized by linguists into various pigeon-holes, but in reality is a continuum. Just as real and imaginary numbers are a continuum of combinations, frequently a sentence is formed to describe a situation with a mix of real and subjunctive moods.
Like when real numbers are applied to the imaginary axis, when normal tenses and moods are applied to the imaginary realm, they take a turn in their directions. The varying and diverse combination of real and imaginary, influences the how the direction changes.
- Imperative: You must have $10 to buy the ticket.
- Cohortative: You must win the game for us.
Propositional disguised as imperative to imbue intensity of belief:
- My credit card must be in the house, or else I'm doomed.
- He must be in the library, but I am not sure.
- He must be lucky. (I know he is not that good. It is just luck)
A mix of propositional and imperative
- He must be lucky. He must have been lucky to have won the race. I really need him to have been lucky. Otherwise, there is no hope for me to win the next race against him.