The word mustn't is a contraction, a shortened form for the verb (or auxiliary verb) 'must' and the adverb 'not'. The apostrophe takes the place of the missing letter 'o'. The contraction functions as a verb (or auxiliary verb) in a sentence. Example:
We must not be late for school. Or, We mustn't belate for school.
adverb
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The apostrophe takes the place of the deleted O in "mustn't" (must not).
part of speech
The part of speech for this particular word is a noun.
adverb
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the part of speech sashay is a averb
Adjective
The word speech is a noun.
Yes, conjunction is a part of speech.
What part of speech is thaw
i want to know what part of speech is camping