The word "can't" is the contracted form of "cannot" or "can not". It means "to be unable to" or "not allowed to". Meanwhile, the word "mustn't" is the contracted form of "must not". It is used to indicate that something is forbidden or, in a tag question, that something is not necessary. When these words are added to a sentence, it changes the meaning or thought of a sentence into the negative form, or in the exact opposite.
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Answer:
The word "can't" is the contracted form of "cannot" or "can not". It means "to be unable to" or "not allowed to". Meanwhile, the word "mustn't" is the contracted form of "must not". It is used to indicate that something is forbidden or, in a tag question, that something is not necessary. When these words are added to a sentence, it changes the meaning or thought of a sentence into the negative form, or in the exact opposite.