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The negative form, don't / doesn't have to, is used to describe something unnecessary.
I don 't have to go to work tomorrow. There's a holiday
Obligation: must / mustn't
The negative is mustn't, with must not is a formal and written form. There is no third
person -s form. Question form is must I, must you etc.
Use must to describe something we personally think is necessary and important
to do.
• There is sometimes little difference between first person I must and I have to. Sorry,
I really have to go now. Sorry, I really must go now!
• In other contexts, there is a difference.
You must be more careful! (= personal opinion of the speaker) We have to wear
safety goggles. (= an 'outside' opinion or rule)
• We usually use the question form of have to for must. Do I have to sit here?
• We use mustn't when we think an action is against the rules. You mustn't throw
things in the science lab! It's dangerous!
• Compare mustn't and don't have to. You mustn't use a pencil. (it's not allowed)
You don't have to use a pen. (it's not necessary- use a pen or a pencil)
Use should, ought to
• to say what we think is right or correct, or is a good idea in your opinion. This is not
as strong as must. You should pay
more attention. You shouldn't talk so much.
• to give advice. I think you should study biology next year.
• in the same way ought to / ought not to has the same meaning as should / should
not.
Use had better
• to say what we think someone should do. There is only one form had better /'d
better. The negative is had better not.
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