Use MUST to command someone from a position of authority. Could be a judge or a parent, etc.
- You must pay the fine within 30 days.
- You must tidy your room if you want to go out later.
Use MUSTN'T to prohibit or warn someone, possibly indirectly (-ve) reporting the rules of authority.
- You mustn't drive without a licence.
- You mustn't do that again!
- You mustn't talk to strangers.
Use MUST and MUSTN'T to talk about an obligation you feel personally.
- I must finish this today!
- They must eat their vegetables too!
- We mustn't be late for the train.
Use HAVE TO in a similar way to MUST, to give obligations from authority - here though it may be indirectly (+ve) reporting the requirement.
- They have to do what the teacher says.
- You have to drive on the left in Ireland.
- I'm sorry, you have to be 18 to see this film.
Use DON'T HAVE TO and NEEDN'T to express a LACK of necessity (NOT a negative order).
- I don't have to do this until tomorrow.
- On Saturdays they don't have to wake up early.
- She needn't come if she doesn't want to.
"Don't have to/Needn't" are VERY different than "Mustn't"
- You mustn't drink that it's poisonous. (Don't drink it!)
- You needn't drink the milk if you don't want to. (You decide to drink or not)
- You don't have to drink alcohol, you know? (You decide to drink or not)
Form of Must
- Has no future form. Use WILL HAVE TO.
- Tomorrow I will have to work all day.
- Has no past form. Use HAD TO.
- I had to pay the fine immediately.
- MUST is not followed by 'TO'
- I must go.
- NOT I must to go.
Form of Needn't
- NEEDN'T is not followed by 'TO'
- I needn't go.
- NOT I needn't to go.