This is from Roget's Thesaurus (NOT the dictionary):
must - verb
I must go ought to, should, have (got) to, need to, be obliged to, be required to, be compelled to.
I think you must know it (must here means certainty). No, this is an opinion. Your opinion, Keannu.
I, Keannu, think you must know .....
It is extremely presumptuous and condescending to say or write "I think you must know..." You may hear it, and you may see it written, but it is what I have said it is.
This from the Oxford Dictionary of the English language:
must 1 modal verb ( past had to or in reported speech must )
1 be obliged to; should (expressing necessity) : you must show your ID card.
Now, Keannu, I think that is about all I can do for you on this thread. I have attempted to explain the use of "must" and I must now beg your leave.
John