He must know

[englishhobby]

Are both contexts possible for the sentence "He must know":

1) A: Do you know the word which means "the act or process of removing water or liquid from a place or thing"? I need it for my crossword puzzle.
B: Ask Jim. He must know. (must expresses strong probability)

2) A: Let's meet after school today. What time are the lessons over?
B: I don't know. - But you must know. You must be more organized. (must expesses duty)

With the verb "know" which of the two functions of must, probability or duty, is more common? (I guess it's the former, but I am not quite certain).
:?:

 

[5jj]

2) A: Let's meet after school today. What time are the lessons over?
B: I don't know. - But you must know. You must be more organized.

The second 'must' expresses obligation. The first expresses logical certainty. However, these two sentences sit ill together.

With 'must know', context will tell you if obligation is intended, as when a history teacher tells his class, "You must know (=must have learnt) these dates by tomorrow".

 

[englishhobby]

However, these two sentences sit ill together.

Which two sentences do you mean? But you must know. You must be more organized. - these ones?

With 'must know', context will tell you if obligation is intended, as when a history teacher tells his class, "You must know (=must have learnt) these dates by tomorrow".

And would it sound natural if at the next lesson the history teacher checked how well they have learnt the dates and said to one of the pupils:
So, you haven't learnt the dates. That's too bad. You must know them if you want to be an educated person. (with a patronizing tone)

 

[5jj]

Which two sentences do you mean? But you must know. You must be more organized. - these ones?

Yes. The first suggests that it is logically certain that the addressee knows, but the second says that the addressee needs to be more organised, suggesting that he does not know.

And would it sound natural if at the next lesson the history teacher checked how well they have learnt the dates and said to one of the pupils:
So, you haven't learnt the dates. That's too bad. You must know them if you want to be an educated person. (with a patronizing tone)

It's possible.

 

[englishhobby]

Yes. The first suggests that it is logically certain that the addressee knows, but the second says that the addressee needs to be more organised, suggesting that he does not know.

What I meant here was not logical cetainty, but kind of strong advice. (You must know. = It is necessary to know.) Can't the verb must be used with such a meaning? I thought the idea of strong advice can be expressed with must (which is stronger) and ought to (less strong)? e.g. You ought to know.