Can we choose "mustn't have" to mean the same as "shouldn't have"?

Question

1 She shouldn't have gone to the disco. (I don't think it was a good idea for her to go to the disco.)2 She mustn't have gone to the disco. (Same meaning as 1)Thanks for answering in another thread! Let me open another thread related to it. Do 1 and 2 have the same meaning?

Kooyeen · Accepted Answer

Heh, Hi Teleostomi,
good question. I think you are interested in the past version of "You must not go to the disco" = "I don't want you to go there".

I'm not sure the past version is "must not + have + past participle".
In other words, are you interested in saying "I didn't want you to go to the disco"? I'm interested too, because I think I don't know how to say that, using an obbligation in the past.
Let's wait for a native...

Yankee · Answer

Teleostomi1 She shouldn't have gone to the disco. (I don't think it was a good idea for her to go to the disco.)

2 She mustn't have gone to the disco. (Same meaning as 1)

Thanks for answering in another thread! Let me open another thread related to it. Do 1 and 2 have the same meaning?

Hi Teleostomi
No, the sentences do not have the same meaning:

1 She shouldn't have gone to the disco. = She went to the disco and I think that was a bad idea.

2 She mustn't have gone to the disco. = Based on the facts I have at my disposal, I can draw the conclusion that she did not go to the disco.

By the way, I would be much more likely to word sentence 2 this way:
She must not have gone to the disco. In other words, I don't think I ever contract this usage of "must not". However, perhaps other people would use the contraction in your sentence.

CalifJim · Answer

My answer is the same as Yankee's, except that I'm fairly indifferent to whether contractions are used or not. must not is a bit more clear than mustn't, but I'd accept either.

I think you and Kooyeen are looking for something like these:

She was not allowed to go to the disco.
She was not supposed to go to the disco.
She was forbidden to go to the disco.
She was told not to go to the disco.
She was wrong to have gone to the disco.

There is no past of "deontic must", that is, the must that is about obligations (rather than about logical conclusions). In some cases you can resort to had to, but in the example above there's a negation that interferes. That's why the expressions above would be used instead.

*She had not to go to the disco.
*She had to not go to the disco.

Of course you could do something like this:

She had to stay away from the disco.

But that's not really the same meaning. It's much weaker and more vague.

CJ

Kooyeen · Answer

CalifJim
There is no past of "deontic must", that is, the must that is about obligations (rather than about logical conclusions).

Wow, I really didn't know that, thanks.

And by the way, I didn't know that "must not" was used for logical conclusions either. I knew about "must", but I remember reading in some grammar book that "must not" was not used for logical conclusions, and that I only had to use "can't" instead. **** grammar books, lol.

CalifJim · Answer

I remember reading in some grammar book that "must not" was not used for logical conclusions, and that I only had to use "can't" instead.

Actually, can't is often the better choice. The substitution of must notis essentially an American invention! I'm told it sounds odd to British ears!

-- Guido didn't finish his polenta.
-- He must not have been hungry.

CJ

What-you-didn't-want-to-know-and-were-afraid-to-ask department: While the must of obligation is called "deontic must", the must of logic is called "epistemic must".

CJ

Teleostomi · Answer

Thanks as always!

So, British people would say "He can't have been hungry." Does it sound British to your ears?

I know "epistemic" and "deontic", I learned here, and I remember perhaps Marius H was using (or you). Even most natives don't seem to recognize them!

CalifJim · Answer

"He can't have been hungry." Does it sound British to your ears?

Nope! Americans seem to accept either one as sounding American!

This is the first time I've mentioned deontic and epistemic, as far as I can remember, so you must not have learned it from me. These are very specialized terms from linguistics (particularly, semantics), so it's no surprise that most people have never heard them. I usually avoid complicated terminology by calling these the must of obligation and the must of logic, respectively.

CJ

Teleostomi · Answer

Nice! (I must not have learned it from you!)

Those difficult words may seem "difficult" at first glance, it's very useful once one remembers the meaning. I like using them!

Can we choose "mustn't have" to mean the same as "shouldn't have"?

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