Obligation: Must vs. Have to

Sometimes, speakers can use ‘must’ or ‘have to’ to mean the same thing that is to express necessity or obligation.

Example: I need to be at the terminal in one hour, so I must / have to go.

In the common usage, speakers use ‘must’ to tell the personal feelings, and ‘have to’ for expressing facts.Example:

• Blink 182 is going to have a concert here. I must see them!

• Students have to wear a uniform in the school.

  We use must for the present or future, but not the past. We can use have to for all tenses. For negative statements and questions, we generally use do / does / did with have to.

Example:

CORRECT	INCORRECT
You don't have to go.	You have not to go.
Do you have to go there?	Have you to go there?
You didn’t have to call me.	You hadn’t to call me.
Why did you have to do that?	Why had you to do that?

Must not / mustn't and don't have to have different meanings. Must not / mustn't mean it is forbidden. Don't have to means it is not necessary.

Example:

• You mustn't cross the road here.
• You don't have to come to my house if you don't have time.