Expressing lack of necessity : do not, have to Expressing prohibition: must not

(a) I finished all of my homework this afternoon. I don’t have to study tonight.
(b) Tomorrow is a holiday. Mary doesn’t have to go to class
Don’t/doesn’t have to expresses the idea that something is not necessary
(c) Children, you must not play with matches!
(d) We must not use that door. The sign says Private: Do Not Enter
Must not expresses prohibition (Do Not Do This)
(e) You mustn’t play with matches
Must + not=mustn’t. (Note: the first “t” is not pronounced)

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