MUST or HAVE TO

There are two modal verbs to show obligation or necessity.

Must Have to  

Must 

 Must is for simple present and usually shows general obligations.

• Travellers must show their passport before boarding a plane. 

• You must use a pencil on the test.

 It is formal and normally used only in writing.

•  All employees must wear proper safety equipment.  

Have to 

Have to is not an actual modal verb, but it is used like a modal.

You must conjugate the verb "have" depending on the time and subject.

•  I have to work tomorrow. 

• She has to work tomorrow.

 It is used in all tenses.

• I had to work last night. 

• She has had to work every day this week. 

"Have to" is less formal than Must and is common in conversation.

 Must not and Don't have to 

 "Must not" and "don't have to" are completely different, and their meanings are often confused.

 "Must not" is a negative obligation and shows something that is prohibited.

            Example:
  You must not eat in the computer room.
(Food in the computer room is prohibited.)

"Don't have to" shows something that is not necessary.

           Example:
   I don't have to work today.
 (Working is not necessary.)


If you need more...
http://www.learnenglish-online.com/grammar/modalverbs.html