Wah How to use "Must and Have to"

Affirmative	Negative	Interrogative
I have to go.	I don't have to go.	Do you have to go?
He has to go.	He doesn't have to go.	Does she have to go?
I must go	I must not go.	Must you go?
He must go.	He mustn't go.	Must she go?

1. We use must or have to to express a strong obligation or necessity base on the rule,an authority or circumstances.
    I have to tell you that I love you. = I must tell you that I love you.
    She has to clean up this room. = She must clean up this room.
    It is close to seven o'clock. We have to go to school.
    The traffic is very bad, we have to take the subway instead.

2. We can also use must or have to with the idea of deduction when we are sure something is true from some evidence or reason.
(More example here: Wah How to use "Must/Might/May/Can't be")
    This meal has to be delicious.
    Mark is a English teacher, he must know a lot of English grammars.

3. Have to can be used in the past or the future.
    Yesterday,I had to work all day. I musted work all day is incorrect.
    Did he have to go to school on last Saturday.
    We will have to get up early.
    Will I have to speak with her?

4. The negative of must indicates obligation that you have to obey or follow.
    You mustn't exceed the speed limit. (It's not allowed.)
    You mustn't park here.

5. The negative of must indicates something that is not necessary to do or follow.
    You don't have to work on Sunday.
    He doesn't have to get up so early.

Answer form:
    A: Do you have to go to college now?  
    B: Yes,I do. or No,I don't.                  

    A: Must you go to college now?
    B: Yes, I must. or No,we mustn't we needn't.

Tag questions:
    John has to keep to the rules,does he?
    John must keep to the rules,mustn't he? (The obligation is being imposed on John.)

See another example:
    Students: Teacher, we must bring our homework next class, mustn't we?
     Teacher: Yes, you must or No,you mustn't you needn't.

When we use must with the idea of deduction, we use do / does / did to make the tag question.
    They've worked out for 5 hours. They must be tired, aren't they?
    He must be an intern, isn't he?
    Mark is a English teacher, he must know a lot of English grammars, doesn't he?