That boy __________ be Jason

[keannu]

For strong negative certainty, only can't is the anwer, But I doubt if "must not" can be used, I don't think so. "must not" should be only for obligation like "You must not be home today = You shouldn't come home today", right?
While "must" can be used for both obligation and certainty. So why is the negative form of "must not" restricted to obligation only?

Q. That boy __________ be Jason. Jason has gone to Japan.


1)can 2)can’t 3)must 4)must not 5)might

[JMurray]

For strong negative certainty, only can't is the anwer, But I doubt if "must not" can be used, I don't think so. "must not" should be only for oblication like "You must not be home today = You shouldn't come home today", right?
While "must" can be used for both obligation and certainty. So why is the negative form of "must not" restricted to obligation only?
Q. That boy __________ be Jason. Jason has gone to Japan.
1)can 2)can’t 3)must 4)must not 5)might

I don't think that "must" necessarily indicates "certainty", so much as an opinion on something that's highly likely: "This must be the bus for the city although it's running slightly early".

And "must not" is quite common in this sort of context: "Oops!… that mustn't have been the city bus, it didn't even stop".

In your example "can't" would be the obvious answer, but I wouldn't be surprised to hear "mustn't be Jason" in conversation.


not a teacher

[SlickVic9000]

(Not a Teacher)

It could be used that way. I imagine the dialogue going like this:

A: Hey, isn't that Jason?
B: Huh? I thought Jason left for Japan last week.
A: Oh, It must not be him, then.

Don't know if that helps or not, but I thought I might try anyway.

[keannu]

For strong negative certainty, only can't is the anwer, But I doubt if "must not" can be used, I don't think so. "must not" should be only for oblication like "You must not be home today = You shouldn't come home today", right?
While "must" can be used for both obligation and certainty. So why is the negative form of "must not" restricted to obligation only?
Q. That boy __________ be Jason. Jason has gone to Japan.
1)can 2)can’t 3)must 4)must not 5)might

I don't think that "must" necessarily indicates "certainty", so much as an opinion on something that's highly likely: "This must be the bus for the city although it's running slightly early".

And "must not" is quite common in this sort of context: "Oops!… that mustn't have been the city bus, it didn't even stop".

In your example "can't" would be the obvious answer, but I wouldn't be surprised to hear "mustn't be Jason" in conversation.


not a teacher

Oh!! My goodness!! I made a serious mistake as I treated "must not" by my student as a wrong answer. So both "can't" and "must not" can work to mean negative certainty. Is it common across all English-speaking countries? If it is true, I should give him more score.
It's all the workbook's mistake, and so far I have been brainwashed to believe "must not" is only for obligation. I feel frustrated!

So which do you use more between "can't be" and "must not be" for certainty?
"It can't be Jason" vs "It must not be Jason"

And "can't have been" vs "mustn't have been"?
"It can't have been Jason" vs "It mustn't have been Jason"

[keannu]

An extra question.
Is it possible depending on context?
a)You must be a good student(obligation) => You have to become a good student.
b)You must be a good student(certainty) => I'm sure you are a good student.

[bhaisahab]

For strong negative certainty, only can't is the anwer, But I doubt if "must not" can be used, I don't think so. "must not" should be only for obligation like "You must not be home today = You shouldn't come home today", right?
While "must" can be used for both obligation and certainty. So why is the negative form of "must not" restricted to obligation only?

Q. That boy __________ be Jason. Jason has gone to Japan.


1)can 2)can’t 3)must 4)must not 5)might

Only "can't" is possible IMO.

[keannu]

Only "can't" is possible IMO.

Can you tell me why your opinion is different from others? I'm so confused.

[BobK]

Only "can't" is possible IMO.

Can you tell me why your opinion is different from others? I'm so confused.

It's a question of context. In nearly all situations, only 'can't' is possible. But even Bhai would accept - I assume ... - a case like this:

'You must choose five members from this list [note: the choices are reported - they're not visible; if they were visible, the receiver of the instuction would know implicitly that he couldn't choose anyone else] to make up the team, and those five must not include Jason (who's in Japan)...' .

Whereas in 'That [indicating someone who's not in Japan] can't be Jason - he's in Japan' the can't indicates logical impossibility, in the extreme case I've given above, the 'must not' refers not to logical impossibility, but to a restriction placed on whoever 'you' is; it'd be quite possible - even probable - to use 'can't' here too: it'd be talking about logical impossibility rather than actual restriction, but it would have effectively the same force.

But this extreme construction of a context that allows 'must not' is just that - an extreme case. Your correction of the student was fine.

b

[Raymott]

Can you tell me why your opinion is different from others? I'm so confused.

"Must not" does not fit that example. The only correct answer (from those given) is 2 (can't). "Couldn't" would also work.

In any case, I don't think the opinions are necessarily different. The first two answers are discussing your opening statement about obligation and certainty and the rest, (which was where your questions are.)
bhaishab's answer addresses the test question (as does mine).

PS: Just read Bob's response. Yes, I suppose you could torture "must not" into place. But the correct answer to questions like this is usually held to the be the most obvious, not just any minimally possible answer. You could also fit "can" and "might" if you had to:
"That boy might be Jason. Jason has gone to Japan. But he could be back by now"

[bhaisahab]

It's a question of context. In nearly all situations, only 'can't' is possible. But even Bhai would accept - I assume ... - a case like this:

'You must choose five members from this list [note: the choices are reported - they're not visible; if they were visible, the receiver of the instuction would know implicitly that he couldn't choose anyone else] to make up the team, and those five must not include Jason (who's in Japan)...' .



But this extreme construction of a context that allows 'must not' is just that - an extreme case. Your correction of the student was fine.

b

Yes, of course I agree.