Student or Learner
When I want to go on a trip all by myself, my father worries a lot about me. Then I say: Dad, you ______ (mustn't, shouldn't, needn't, can't) worry about me, for I've grown up now.
In this case, needn't and shouldn't work well. Could we also say 'mustn't' (and 'must not' for Americans)? Does it sound impolite?
Thank you in advance.
<mustn't> is stronger than <shouldn't>. Uncontracted <must not> is even stronger and would not be used in NaE as a normal neutral. <don't have to> would likely be more common for NaE and it serves as a normal neutral.
<can't> would be used as a relatively strong admonishment where the father has an annoying habit of overworrying. <you can't keep worrying about me. I'm an adult/grownup>
Intonation would also play a big role here in nuancing these modals/semi-modals further.
<needn't> and <don't need to>, the latter the more common choice for NaE are relatively neutral.
Sure you can. Intonation can mean a great deal. Remember too, that the overall context is vitally important. If the situation is that the son is worried about his father overworrying, possibly because Pa has a heart problem and the doctor has told him not to worry as it cause him greater anxiety than it does most people.
In such a situation, the strength of 'must' has shifted, hasn't it, to a total concern for the father, not as any sort of admonishment by the son for this annoying habit of the father. In such a case, there could be a further "weakening/softening of the situation, as in,
"... I'm grown up now and you know I can take care of myself, just as you did at my age. I have your good common sense and you know that I never take risks, ....