Question

Have you ever came to know about the word " Mustn't " while reading books or while conversation??

It's is the least used modal​

Answer 1

Hi mate.........

Actually "Mustn't" is the short form of "Must Not" which is commonly used in books, texts and usual conversations.

It is just as same as we use havn't, shouldn't, couldn't and so on.....

Hope this will prove helpfull to you....