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ESL forum > Grammar and Linguistics > can t vs mustn t for prohibition    

can t vs mustn t for prohibition


can t vs mustn t for prohibition
Dear All !

I m in doubt. 

What is the difference between can t and mustn t for prohibition?

Thanks a lot in advance!

24 Jun 2012      

Korea, South

You use "Can t" when it is not possible to do it; Often use colloquially, and sometimes it s a question of ability.  For instance, I can t run that far.
You use "Mustn t" when it s an imperative not to do it (an order). For instance, you mustn t text while driving otherwise you will get a ticket.  

24 Jun 2012     


Hi Nan!
When I was at the teacher trainning college my friends and I used a rather useful way of remembering: can t we associated with "inside"= It s you who decide, you are the judge, while musn t we associated with "outside"=somebody else decides for you, somebody else is the judge.
I hope this explanation works for you too.

24 Jun 2012     


24 Jun 2012     

United States

can t = you are unable to do it
mustn t = you are not allowed to (or shouldn t) do it

25 Jun 2012     

United States

Good answers so far, but I need to add something.
We also use "you can t" when referring to things that are illegal, or otherwise against the rules.
You can t smoke in restaurants in most of the US.
The sign says that you can t park here.
You can t wear flip-flops to school.

25 Jun 2012     



Can t = negative when must is used in the sense of obligation / prohibition.

You must do your homework on time. / You can t turn in your homework incomplete.

Mustn t = negative when must is used in the sense of probability.

Steve must be home now. / Steve mustn t be at his office; I heard he s sick in bed.

Have a nice day!

Zailda Coirano

25 Jun 2012     


according to me can t is used to show an inability, e.g: i can t do this work . while mustn t is used to show prohibition, e.g: you mustn t be late again!

11 Jul 2012