Do you say 'mustn't' in conversational English?

Usually in casual English we just use “don’t”. “Mustn’t” is a bit formal and it’s mostly seen in academic or literary settings

I do, but I’m pretentious and erudite. In a more casual conversation, I’d say “shouldn’t” or “can’t”.

It's used all the time: I mustn't have had my phone switched on. She mustn't have known that he was going to be there. They mustn't have covered this properly in class. You mustn't ever say that again. He mustn't've understood. You mustn't do that. They mustn't know how to turn it on.

Mustn't grumble

I don't because I'm not a 19th century English schoolmarm.

Yeah but I intentionally like to use uncommon words because it's fun so you probably shouldn't use me as an example

If we realise that we forgot to do something (like lock the door or turn on the dishwasher, etc), we might say: >"I must not have done it." where "must" means something like "logically necesarry". So it means something like "I have realised/deduced that I didn't do it." Which we might shorten this to: >"I mustn't have done it." Or, without realising it, we'd perhaps contract it further to: >"I mustn't've done it." That last one looks silly when I write it down, but we probably do speak like that.

I find that people who use it are usually doing so for comedic effect or exaggeration. It just sounds silly for whatever reason It may seem strange, but "must not" is more common because we use "must" as a kind of force. There is no good synonym for it. When you contract words, you tend to lose emphasis, so "mustn't" defeats itself in that regard. "You should not touch the grey slime" is weaker than "You must not touch the grey slime", but more forceful than "You mustn't touch the grey slime."

I think people usually say "can't" or "better not" instead. You mustn't do that. / You can't do that. / You'd better not do that. "better not" is more of a threat or admonition for something that is technically possible to do, but will have negative consequences. "can't" is used when something is either against the rules or not possible.

I will not that mustn't is pronounced mussn't, in case you've only encountered this in writing. As for usage: maybe occasionally, but pretty much only if one chose to be extra formal (which ironically might be most be commonly for comedic effect).

<American English> "shouldn't", "don't", "can't", or "aren't allowed" You mustn't park there. \-> You can't park there. \-> Don't park there. \-> You shouldn't park there. \-> You aren't allowed to park there.

nobody uses mustn’t unless you’re trying to be funny. or like some people here say, being extremely formal, and usually in writing if anything.

We say it. "Must'nt" is pronounced "mussint." It is NEVER written as a contraction. NEVER. And, in my opinion, "mussint" is a little bit formal, and a lot archaic. "Must not" is good enough for casual and formal speech and written communication.

Muss + n + glottal stop.

Australian, it's actually extremely common here.

As an Australian I break it up. Not really common here I think

No. "Must" is a pretty rare part of my vocabulary (American). I use it in the construction "X must be Y" in a situation where I'm saying "Huh, well I guess if X isn't A or Z, the only option must be Y." It stresses the the truth of the statement where I'm more confident than just "I guess X is Y." This is the only example I can come up with for when I'd use the word must, and in this case, if I'm negating it, I would be stressing the un-truth, and it doesn't make sense to use the contraction "mustn't" because it skims over the "not" aspect. My guess is that "has to" has largely replaced "must" to express necessity (at least in the States), and you'll hear a lot of "hasn't" and "haven't" but not "mustn't." Using "mustn't" sounds rather prim and quaint to my ears—a word someone's British grandmother might say.

Very rarely, and usually as a joke. “Shouldn’t”and “better not” are much more common. I think English people say it more than we Americans do.

as an american speaker, i'll use **mustn't've** or **must've** but almost never will i say mustn't

Used enough to be worth learning imo. All these people making generalist statements about it being formal and never used are wrong. That may be their experience but taking it to represent English speakers as a whole is naive. I use mustn’t all the time, in very casual conversational English, and I by no means work in a highly intelligent field.

USA. Never heard someone say that out loud.

It’s used in some dialects, but American English notably doesn’t use “mustn’t” outside of technical language and when feigning accents. Generally we’d say “can’t”, “shouldn’t” or “don’t” instead.

I usually use "don't you fucking dare Greg..."

I hear "mustn't" a lot from UK English speakers and Australians whose English has been influenced by UK English (like British parents or time spent in the UK). I can say confidently that I never say "mustn't" though. It just doesn't come out of my mouth naturally. Oddly, "must not" is more natural to me than the contraction.

As pointed out, it depends on the dialect. In british english speech if you *don't* contract "must not" to "mustn't" the sentence sounds emphatic or like a command. And it's usually pronounced 'mussent', the middle t is silent in the contracted form.

Yes, we do.

A good rule of thumb is that we use them all frequently. Wouldn't Couldn't Shouldn't Mustn't Isn't Then, when you want to stress the point or to have a more formal approach, you'd say, "That must not be right/correct"

Yes, I use it a lot, especially for past tense uses. 

Worth noting, in English one typically doesn’t negate “must”, rather you negate the other verb. E.g. “You must lock the door when you leave.” - It is required that you lock the door. “You must *not* lock the door when you leave.” - It is required that you not lock the door. Other languages let you negate the compulsion in must, allowing you to say “it is not required/it is optional”. In English you might say “You don’t need to lock the door.”

I occasionally say or hear it in it's uncontracted form. "I must not have brought it. I thought I did." "She must not have remembered that we were meeting today."

I do because I’m just a silly goofy guy

I hear it used very rarely if ever

I haven’t heard it much at all as an American, but if I claim we don’t use it here I’ll probs get downvoted lmaooo. I’m sure some dialects do use it more often in the country and in the world in general !!

I (US English) would say “shouldn’t”. I do say “mussnotta” for “must not have”, but never “mustn’t”.

Yes

East coast USA, no one uses it. It sounds strange to me, like old timey English.

I use "must not've" a lot. Like I must not've turned down the stove so the pot boiled dry. I wouldn't use musn't at all but my brain translates it to must never. Like you musn't do that would mean you must never do that.

Sounds like a British thing in a proper speaking context — like at a school where they teach you manners. I don’t think Americans, like myself use “mustn’t” too often (if at all). We do use “must” sometimes.

Mustn't is common but mayn't isn't.

Never in my life as an American

As an American, no, unless I was trying to sound like a person in a snooty British drama as a joke.

As an American, it sounds very Jane Austen.

Only when trying to sound formal or archaic for a comedic effect

I'm American and have never said or heard that in any natural way. If I was making fun of someone for being posh or something maybe. It's definitely more British and to me sounds 19th century, although I guess it's still somewhat common in the UK

It depends on the context. In North American English it's uncommon to be used as a proscription like they would in Britain (ie "You mustn't be rude to your teacher"). In NA English we would use the word "can't" or "shouldn't", depending on the specific context. However, the word mustn't finds a little more use in NA English not in the form of proscription, but of supposition. "It mustn't have been serious, because the school didn't send her home". Even in this case though, people will often say "must not" instead of contracting the words together.

We just say "shouldn't".  We never say mustn't except to sound funny. We also rarely verbally say "must not".  "You can't do that!" "You shouldn't do that."  Only time I can think of must is "I must have misheard you" or "I just have just missed you."  But very rarely "I must not have heard you". 

Shouldn’t is likely the most common usage (must is like a required action, so should - which expresses compliance - is closely related). I never say mustn’t and I never hear it unless I’m reading or watching a period piece, meaning like the 19th century or something before that.

American English speaker, I rarely use 'mustn't' and when I do use it I don't contract it and say 'must not' instead. Otherwise I phrase it other ways depending on context, like 'shouldn't/couldn't/don't/can't' etc.

i don’t…but when running phrases through my head, i say “must have,” as in: she must have missed the train; i must have forgotten that part of the story. i do seem to use it in negative way, as in something not done. “mustn’t” is “must not.” is “have” implied? must ponder this. mustn’t get distracted. 😀

That's a word?

very common in International English, pronounced 'mussent' sorry no IPA

It’s quite common where I’m from, yeah. Particularly with older people.

The Brits use it. Americans will use it when imitating Brits but not at any other time.

“Mustn’t grumble.” Wallace and Gromet

I use it, but I hear "must not" more often. As a 64 year old, I've come to suspect a generational shift with these things.

I never say must or mustn't if I can help it. Mostly because telling someone what they must or mustn't do is a quick way to get them to do the opposite. Shouldn't, wouldn't, is not advisable....

As an American, sounds overly fancy.

I don't. But I don't use "must not" much either. I usually will say shouldnt/should not or can't/can not instead depending on the context

This one is very popular in the UK and apparently AUS. Not at all used in the US. I’m not sure about CAN and NZ.

Whelp. Here is my thought. Must=things that you with no choice to do without consequences. Should=you have a choice to do or not. Ex. You must have a drivers license to drive. You must wear a seatbelt or face a fine. No choice. You shouldn’t drive and drink coffee. You could get hurt. It’s in your best interest to not do this but not against the law. Choice.

I’m an American English speaker and I very rarely use this word in conversation

In the US, we might use it in formal situations, like during meetings, presentations, formally speaking with a superior, etc. We broadly associate it with the way the British speak, which we associate with proper or formal speech. In such situations we are probably emphasizing something that should not be done. Otherwise it's fairly rare, used in combination with further contraction in past predictive sentences, as another commment has said.

I can honestly say I've used the word verbally. I've even heard the word used. Until now, I've never seen it written, and it just looks weird to me...

In the USA, I say Mustn't've, which, when i say it, is indistinguishable from "musn't of" If I get home and can see the light on from the sidewalk: "Shit, I mustn't've shut the light off..."

"Mustn't" is fine. "Must not " for emphasis 

"Mustn't" is fairly common in British English, but pretty rare in American

til i’ve never seen the word “mustn’t” spelled out

Mustn’t sounds old fashioned or possibly British. Not sure if it’s still used in British English. for example instead of “You mustn’t be late” We’d say “Don’t be late” or “you shouldn’t be late.”