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First, let me give you two examples:
She has a difficult job. She must work hard.
She has a very difficult life. She must be patient.
Since "must" can also be used for meaning "most likely" instead of its first meaning. Doesn't it cause ambiguity in these kinds of sentences? If I am meaning she most likely works hard or that she is most likely patient, can't it be understood as like she should work hard or she should be patient (I know that "must" is stronger than "should" by the way)?
Sure, but there are hundreds of other English words with similar ambiguity. Consider all the various meanings of "have" and "get", for example. English speakers simply learn to pick the most common or likely meaning from the context.
For example:
He must be a good student.
Without any surrounding context, the most likely meaning is that I think he is a good student, not that he has to be a good student. However, that could change with context.
He always gets good grades. He must be a good student.
He isn't serious enough. He must be a good student. Otherwise he won't graduate with the rest of his class.
In both of these examples, the meaning of "must" should be obvious, even though the basic sentence is the same.
You must work hard at that job.
That sentence can mean a number of things, for example:
"No doubt you do work hard at that job."
"That job demands hard work from whoever does it."
"You should work hard at that job!"
You must understand the context to determine the meaning.
