what's the difference between shall and should in this sentence

[chance22]

I'm confused when completing the following sentence: He ____ be punished if he disobeys the law.
Why should "shall" be used here instead of "should"? What's the difference between the two words? Could you explain?Thank you.

[Rover_KE]

Who says 'shall' is the only correct answer?

They are both possible.

Rover

[chance22]

Who says 'shall' is the only correct answer?

They are both possible.

Rover

Then I'm relieved. As the key to this multiple choice Ex suggests, I'm supposed to use "shall", but I don't see the difference between them. Thank you. I think I'll just ignore the key.

[bhaisahab]

Then I'm relieved. As the key to this multiple choice Ex suggests, I'm supposed to use "shall", but I don't see the difference between them. Thank you. I think I'll just ignore the key.

There is a difference of meaning but, as Rover said, both are possible.

[chance22]

There is a difference of meaning but, as Rover said, both are possible.

Then could you tell me the difference? Does "shall" indicates an event that is to happen naturally as a result while "should" means a "must"?

[Raymott]

Then could you tell me the difference? Does "shall" indicates an event that is to happen naturally as a result while "should" means a "must"?

No. 'Shall' means 'will' (if you'll allow a simplification). "He will be punished."
"Should" means "ought to be". "He should be punished, but he might not be." It's hard to explain "should", but you should try to get its meaning. If I said he 'should' be punished, it means that I believe that the fairest outcome is that he be punished; it's in society's interests that he be punished. A just society based on law and order would punish him.
It doesn't mean he must be punished.

[SoothingDave]

"Shall" (in the 2nd or 3rd person) is a command. It's found most often nowadays, in AmE anyway, in the law. "The secretary shall determine..." "The sheriff shall issue..."

There is no contingency or will involved.

[chance22]

Thank Raymott and SoothingDave for the explanation. I'll read more and get the exact meaning of the words in different contexts