can't or mustn't ?

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carolineR

Senior Member
France
when I was younger decades ago, I was taught "can't" was the opposite of "must" when both expressed probability
e.g. : Where's your mother ? - She must be home by now/ She can't be home yet.
Now, we are told the opposite : "modern" grammars all insist "mustn't" must indeed be used
Where's Mum ? - she mustn't be home yet.
What do you think ?
 
  • roxcyn

    Senior Member
    USA
    American English [AmE]
    Personally I don't use it like that, but according to my grammar book it says it is fine.
     

    roxcyn

    Senior Member
    USA
    American English [AmE]
    A Comprehensive English Grammar (1976)

    Here are some explanations it gives:

    "Must has an imperative quality suggesting a command or an obligation."

    Example: You mustn't walk on the grass.

    "Must implies a logical conclusion, a strong likelihood, something that seems the only reasonable explanation"

    Example: Mr. Green mustn't be a home.
    (mustn't can be replaced with can't as you said in your question)

    "When the meaning of must is 'logical conclusion', etc, the usual negative is cannot (can't)"

    Examples:
    If you said that, you must be mistaken.
    If you said that, you can't be telling the truth.

    "When the negative of must has the meaning 'it is not necessary', 'there is no obligation', need not (needn't) is used"

    Example:
    Must I answer all the questions? No, you needn't answer them all....
     

    carolineR

    Senior Member
    France
    Example: Mr. Green mustn't be a home.
    (mustn't can be replaced with can't as you said in your question)
    "When the meaning of must is 'logical conclusion', etc, the usual negative is cannot (can't)"
    If you said that, you must be mistaken.
    If you said that, you can't be telling the truth.
    Yes, Thank you roxcyn, this is exactly it:
    my question is : do you ever use mustn't in that sort of situation or do English-speakers systematically choose can't ? When you speak/ write ?
    Is there a difference in AE/BE ?
     

    roxcyn

    Senior Member
    USA
    American English [AmE]
    Okay, no I don't use "mustn't" when I speak and write. Maybe some BE or AusE speakers can tell you if they use the structure :).
     

    carolineR

    Senior Member
    France
    I use it a lot, as in "He mustn't understand". "He mustn't know what he's talking about". It seems always to refer to guys, in my parlance!;) Just kidding, but I would say it's more informal, as I think many contractions are.
    What do you make of this, (from another post), then ? :)
     

    fenixpollo

    moderator
    American English
    As an AE speaker, I view "mustn't" as strictly BE, or old-fashioned AE. I don't think anybody who was born before the war could credibly use it in conversation... and I don't remember having seen in written before now.

    Cheers.
     

    LV4-26

    Senior Member
    I'm not a Brit but I sense a difference between
    (1) She mustn't be home yet and
    (2) She can't be home yet

    To me, (1) means she's probably not home yet and (2) means she's certainly not home yet.

    But I'm not sure. Would Mc Enroe have been fined if he'd said "you mustn't be serious!"? :)
     

    fenixpollo

    moderator
    American English
    I hear a different meaning in your examples, LV:
    (1) No! It would be horrible if she were home already! She'll discover my secret!
    (2) It isn't possible that she's home yet; I don't believe it!

    I think MacEnroe would have been fined in either case. ;)
     

    fenixpollo

    moderator
    American English
    Well, I interpreted your explanations to reflect degrees of probability, not degrees of believability or emotional involvement. If there was emotion behind your example, then you're right: we don't differ on (2). :)
     

    marget

    Senior Member
    What do you make of this, (from another post), then ? :)
    If I say "he mustn't know what he's talking about", I mean "he probably doesn't know". "He can't know what he's talking about", for me, means that he just about absolutely doesn't know. Another example might be "You can't be serious", which I would say if I meant that it is not possible that you're serious. If I were to say "You mustn't be serious, I'd mean that you're probably not serious. Yet I probably wouldn't say that. I think I'd say "You must be joking/kidding".
     

    carolineR

    Senior Member
    France
    Well then, can't and mustn't do cohabit, then... and are different in meaning. I wasn't sure I had ever seen mustn't used in that sense.
    Interesting
    Thank you everybody :)
     

    panjandrum

    Lapsed Moderator
    English-Ireland (top end)
    Having just checked the dictionary for the related thread about mustn't, I have my finger in the place where mustn't is explained. I quoted the mustn't & past definition in the other thread - here is the mustn't & present:
    10 b. Referring to the present: is (am, are) certain not to, may be presumed not to, cannot.
    I hear a similar softer sense to mustn't. Can't seems to be a statement of the absolute. In these examples, mustn't mostly means may be presumed not to, to me.
     

    panjandrum

    Lapsed Moderator
    English-Ireland (top end)
    No we're not.
    There seems to be a tendency to overlook the probability sense in much of the earlier discussion, hence the need to draw attention to the softer meaning.
     

    LV4-26

    Senior Member
    former chomsky advocate, CarolineR's initial question was about probability.

    marget and panj, are you suggesting in your respective posts #13 and #15, that you basically agree with my post #9? :) (mustn't be home = is probably not home).
     

    panjandrum

    Lapsed Moderator
    English-Ireland (top end)
    [...]
    marget and panj, are you suggesting in your respective posts #13 and #15, that you basically agree with my post #9? :) (mustn't be home = is probably not home).
    Yes - I was rushing and didn't make that clear. I was hoping that you and marget would recognise yourselves in the word similar :D

    Must is funny. As fca suggests, depending on context it can be either an order or a strong recommendation.
    You must see Hogswatch tonight.
     

    Cadet Rousselle

    Senior Member
    English
    when I was younger decades ago, I was taught "can't" was the opposite of "must" when both expressed probability
    e.g. : Where's your mother ? - She must be home by now/ She can't be home yet.
    Now, we are told the opposite : "modern" grammars all insist "mustn't" must indeed be used
    Where's Mum ? - she mustn't be home yet.
    What do you think ?
    As an AusE speaker here's my views.

    The statement "She can't be home yet" has two meanings depending on the tone you use typically.

    It can mean:

    1) It is totally impossible for her to be home now. (maybe it's 4:55pm and she finishes work at 5:30pm)

    2) You expect her to be home, discover she is not, and think to yourself "she can't be home yet".

    This second meaning is the same as one of the meanings of "mustn't be home yet"

    "mustn't be home yet" is typically:

    1) see 2) above.

    2) It can be used theoretically like the imperitif "she mustn't be home yet, stop her!!!" but it's very very rare and non-standard IMHO.

    My opinion, they are synonyms, use mustn't here like can't is used in 2), or just stick to can't, either way you'll be fine.

    ne t'inquietes pas. :)
     

    LV4-26

    Senior Member
    So, could you tell me your conclusion?

    Whether "mustn't" in the sense we're dealing with here [epistemic use] is really used now.
    We both know what epistemic refers to, as we've both read MrPedantic's post in the other thread. ;) For those like me who hadn't got a clue what on earth it could mean before reading it, click here.

    I'm not sure we've actually reached an ultimate conclusion. The main answer is here, in panjandrum's post
    Can't seems to be a statement of the absolute. In these examples, mustn't mostly means may be presumed not to, to me.
    Therfore, if you're absolutely sure mom isn't here, you'll use can't. Also note that, according to fenixpollo, there may be a certain degree of emotional involvement in this can't. (I can't believe it).

    If you just presume it, you might use mustn't. Some people do, others don't.

    I wrote might above, because mustn't, in the epistemic sense, doesn't seem to be used that much. What led me to this conclusion is that, even though Panjandrum and marget think it's fine.....
    - you don't find many instances of it on the Internet
    - when I used it this way, fenixpollo spontaneously interpreted it in the deontic sense (obligation).

    So, what happens when people are, say, 60% to 85% sure mom isn't home
    and they think "may not" doesn't convey a sufficient degree of probability?

    I believe they prefer something like "I don't think mom is home yet".
    Or, more colloquially, "I don't reckon my mom's got in yet".
     

    carolineR

    Senior Member
    France
    Thank you LV4-26 for that nice recap' :cool: indeed !
    Now I feel like complicating things a bit further
    If (few) natives use mustn't + present infinitive in the sense "may be presumed not to", what would they make of must + past infinitive ? Would anyone ever say : the children mustn't have stolen the jar of marmalade ?
    In other words
    what would the opposite of Alice must have been killed in yesterday's car accident
    Would it be
    She can't have been killed in it (impossible)
    or can it be -at least for some native-speakers -She mustn't have been killed in it ? :)
     

    panjandrum

    Lapsed Moderator
    English-Ireland (top end)
    In other words
    what would the opposite of Alice must have been killed in yesterday's car accident
    I understand in two ways, depending on context.

    I am deducing from the evidence that it's highly probable that Alice was killed in the accident - opposite sentence uses mustn't.

    I am making an assertion that there is no chance that Alice survived the accident - opposite sentence uses can't.
    Would it be
     

    carolineR

    Senior Member
    France
    Thank you Panjandrum :)
    and you would actually say a sentence like "Alice mustn't have been killed in the car accident" ? I mean it is used ???
    the reason I am insisting is
    1. in 50 years of studying Engklish, I don't think I've ever heard/read it,
    2. it is the latest craze among inspectors in France : one mustn't teach can't is the opposite of must (epistemic use, of course), one must teach mustn't...
    and it sounds just weird to my ears :)
     

    winklepicker

    Senior Member
    English (UK)
    you would actually say a sentence like "Alice mustn't have been killed in the car accident" ? I mean it is used ???
    the reason I am insisting is
    1. in 50 years of studying Engklish, I don't think I've ever heard/read it,
    2. it is the latest craze among inspectors in France : one mustn't teach can't is the opposite of must (epistemic use, of course), one must teach mustn't...
    Dear carolineR,

    It sounds completely weird to my ears too. I don't think I've ever heard mustn't used in this sense. I would always expect to hear "Alice can't have been killed in the car accident" (though that is an unlikely scenario!). "She can't be home yet" is certainly what I'd expect to hear.

    Theoretically it might be possible, but at least in this part of the world it is never or almost never used. I think your inspectors have got it wrong - at least as far as common BE usage is concerned.

    A sympathetic Winklepicker
     

    pyan

    Senior Member
    English, UK, London
    Thank you Panjandrum :)
    and you would actually say a sentence like "Alice mustn't have been killed in the car accident" ? I mean it is used ???
    the reason I am insisting is
    1. in 50 years of studying Engklish, I don't think I've ever heard/read it,
    2. it is the latest craze among inspectors in France : one mustn't teach can't is the opposite of must (epistemic use, of course), one must teach mustn't...
    and it sounds just weird to my ears :)
    I would use "mustn't" in the way given in your example.
    The washing machine is still full of dirty clothes. I mustn't have switched it on last night.
    Andy didn't get the card until yesterday, so Ingrid mustn't have posted it in time.
     

    JamesM

    Senior Member
    Thank you Panjandrum :)
    and you would actually say a sentence like "Alice mustn't have been killed in the car accident" ? I mean it is used ???
    My answer is "yes". For example, let's say I'm watching a movie where Alice is ostensibly killed in a car accident. A few scenes later I suddenly see Alice alive and walking around. I could say to myself:" Alice must not have been killed in the car accident!" This means it is logical to deduce that she was not killed since I now see her walking around.

    (In my experience, "mustn't" as a contraction is not used as often in American English as it is in British English. I think we AE speakers tend to say "must not" when talking about the logical deduction or the probability.)

    As another example, let's say that I see Linda, a co-worker, sitting in the conference room alone at 3:00 p.m.. I know that there was a meeting scheduled for 3:00 p.m. but I also know that it was cancelled. I could say to someone standing nearby: "Linda must not have gotten the message about the meeting being cancelled."
     
    Last edited:

    carolineR

    Senior Member
    France
    My answer is "yes". For example, let's say I'm watching a movie where Alice is ostensibly killed in a car accident. A few scenes later I suddenly see Alice alive and walking around. I could say to myself:" Alice must not have been killed in the car accident!" This means it is logical to deduce that she was not killed since I now see her walking around.

    (In my experience, "mustn't" as a contraction is not used as often in American English as it is in British English. I think we AE speakers tend to say "must not" when talking about the logical deduction or the probability.)
    Yes, i noticed that.

    As another example, let's say that I see Linda, a co-worker, sitting in the conference room alone at 3:00 p.m.. I know that there was a meeting scheduled for 3:00 p.m. but I also know that it was cancelled. I could say to someone standing nearby: "Linda must not have gotten the message about the meeting being cancelled."
    how interesting. thank you for that detailed answer :thumbsup:
     

    LV4-26

    Senior Member
    I think the main issue in this thread may be the difference between opposite (a word you used in your opening post) and negation..

    Basically, I'd say mustn't would be the negation of must while can't would be more like the opposite of must.

    Of course, keeping in mind that mustn't doesn't seem to be used in this sense (low probability) in AmE.

    What is the opposite of white? Is it black or not white? :)
     

    sound shift

    Senior Member
    English - England
    I don't think I would say "Alice mustn't have died in the accident." I think I would say "Alice must have survived the accident."
    I don't think I would say "I mustn't have switched the washing machine on last night." I think I would say "I must have forgotten to switch the washing machine on last night."
     

    johndot

    Senior Member
    English - England
    There are some good points raised in the posts from LV4-26 and sound shift (# 34 and 35), and I think it’s worthwhile to introduce a slightly tangential issue which has a similarly incongruous aspect.

    In this brief dialogue:

    A. Must I do it this way?
    B. No, you mustn’t.

    does B mean...

    1. You are obliged not to do it that way.
    2. You are not obliged to do it that way.

    ...?

    It seems to me that the negative of must is so ill-defined that anyone who uses it mustn’t ever be sure that the person he’s speaking to has understood it in the way that it was meant (given that the speaker himself knew what he had in mind).

    To paraphrase LVA-26, is not-black white, or is black not-white? The only conclusion I can reach is that it’s a grey area.
     

    giovannino

    Senior Member
    Italian, Neapolitan
    I was a bit disappointed to find out that even A Comprehensive Grammar of the English Language fails to make a semantic distinction between can't and mustn't:

    "The auxiliary negation of must in the logical necessity sense is usually achieved through can't:

    They must be telling lies ['It is certain that they are telling lies']
    ~They can't be telling lies ['It's not possible that they are telling lies']

    But must not and mustn't occur occasionally in this sense in AmE and, still more occasionally, in BrE. This use (instead of can't) seems to be gaining favour."

    From what I've gathered in this thread those native speakers who do use mustn't/must not in this sense make the difference mentioned by LV4-26 in posts #9 and #23 and confirmed by marget and panjandrum:

    LV4-26 said:
    if you're absolutely sure mom isn't here, you'll use can't.
    If you just presume it, you might use mustn't.
     
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