Here are the main points about must and have to.
1) Both 'have to' and 'must' are used to express obligation.
You must stop eating so much.
You have to take some exercise.
2) If something is an opinion or a rule you've made yourself, you're more likely to use 'must'.
I must remember to buy a birthday card for Alice today.
3) The negative forms 'mustn't' and 'don't have to' have very different meanings.
You don't have to stop celebrating Christmas every day. (It’s not necessary to stop celebrating Christmas every day.)
You mustn't eat all that fatty food. (You must not eat fatty food. Don’t do it.)
4) If you mustn't do something, it is necessary that you don't do it.
You mustn't eat all that fatty food.
5) If you don't have to do something, it doesn't matter whether you do it or not.
You don't have to finish it today, you can give me it on Monday.
6) Have to + infinitive (without ‘to') is the correct form.
You have to go
I have to remember
Karen has to leave before 6 o'clock
7) 'Have to' is more common in questions.
Do I have to pay now?
Does Kori have to play his music so loud?
单词：
expires：到期；有效期 officially finishes, usually after a particular date
grumble：抱怨 can’t complain; not bad (said after someone asks 'How are you?')
obligation：义务
pronounciation tip:
关于must的读音
如果must后面跟的是辅音，则must中的t是不发音的，例如：
I mus(t) go
You mus(t) come
如果must后面跟的是元音，则must中的t是发音的，但是会和后面的单词连在一起读，例如：
I must open
You must arrive
The 't' is there, but it joins on to the word afterwards.
'Mus topen'
'Mus tarrive'
If you must do something, it is necessary for you to do it, but this is often your opinion or a rule that you have made yourself.
If you have to do something, it is necessary for you to do it. It’s a law, an obligation or a fact.
If you don’t have to do something, it isn’t necessary to do it, but you can if you want.
If you mustn’t do something, it means ‘don’t do it’. It is necessary not to do it.