Mustn’t – needn’t – don’t have to

Must	Must – Have to	Mustn’t – needn’t – don’t have to	Didn’t need to / Needn’t have done
Obligation – necessity – duty You must stop when the traffic light is red.	We use must when speaker decides that something is necessary I must find a hotel to stay at tonight. (I have decided it by myself)	Mustn’t = it is forbidden to do sth You mustn’t smoke here!	Didn’t need to + infinitive = it was not necessary to do sth They didn’t need to buy more vegetables for the soup (= it wasn’t necessary for them to buy vegetables, so they didn’t)
Prohibition You must not (cannot) park here.	We use have to when somebody else other than the speaker has made the decision I have to wear a uniform at work. (Somebody else has made the decision)	Needn’t = it isn’t necessary to do sth She needn’t / doesn’t need to / doesn’t have to to lose weight. She is slim!	Needn’t have + past participle = it was not necessary to do sth, but it was done You needn’t have bought me a gift (= it wasn’t necessary for you to buy the gift, but you did)
Strong advice You must behave good at school.
Logical assumptions a) Positive You must be ill. You have a temperature! b) Negative (can’t) You can’t be ill. You don’t have a temperature.
Only for present tenses! Present Simple – must / have to Past Simple / had to Present Perfect – have / has had to Future simple – must / will have to

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