Simple question

Uh, yes, but I'd like to see it in context, Taka.

Maybe:

Bank-robber: OK, open the safe and hand over the money.
Terrified bank clerk: But I don't know the code!
Bank-robber: You must know it!

Or:

Concert-goer #1: That theme in the 2nd movement...It reminded me a little of the 1st subject in Barber's Adagio for Strings.
Concert-goer #2: Barber's Adagio for Strings? I don't know it, I'm afraid.

(from a composition of a student of mine)

When I lived in Japan, I was overconfident that I must know everything about Japan, just because I was a Japanese. But I was wrong.

I don't know why, but I feel that "must" is strange...

I was convinced that I knew everything about Japan ...

"must" is strange, yes.

I think it's got something to do with being in a subordinate clause introduced by a past tense. The order of the clauses should probably be changed if "must" is to be used. And then a few words of padding might be needed to soften the blow of the necessary switch from past to present, "must" hav

Jim, is the past tense of "I was a Japanese" itself wrong grammatically?

Not at all. I am more used to hearing the adjectival forms of nationalities and ethnicities. The "a" bothers me, but not the "was".

He was Japanese, Italian, Swedish, French, Chinese, etc.
Not so often - He was a Japanese, an Italian, a Swede, a French (Frenchman?), a Chinese.

Some people avoid "a Jew", for example, as possibly offensive, preferring "Jewish". I can't

I see.

Thank you all!

I find the strangeness only in 'overconfident', which (as CJ says) pre-empts 'I was wrong'.

I wouldn't be perturbed by:

'When I lived in Japan, I was confident that I must know everything about Japan, just because I was Japanese. But I was wrong.'

—though this sense of 'must' is almost implicit in 'confident'.

'Must' seems permissible in a past subordinate

Thanks again, MrP!

Hello again Taka

I was intrigued by your original question ('Is it possible to use "You must know it" to mean..."I'm sure you know it"?').

It seems that this nuance of 'must' has quite a long history. In Chaucer's Legend of Medea (c. 1385), you can find:

'Tho gan this Medea to him declare
The peril of this case,...and in what disjoint
He mote stande.