Must, Have To, and Need

The verbs need, have to, and must are all synonyms of one another and are used to mean that something is necessary or required. But they each have slightly different connotations and uses.

Need, when followed by to and a verb, is the narrowest of the three verbs. It is most often used to say that an action should be done:

I need to wash my dirty clothes.

It is also used to say that it is very important and necessary for someone to do something :

I need to call my mom and find out how my dad's surgery went. [=It is necessary and important for me to call my mom and find out how my dad's surgery went.]

She didn't want to go into town, but she needed to buy groceries. [=It is important that she go into town because she will not have groceries otherwise.]

Must and have to are much broader. They can both be used to say that something is required or necessary:

We must go to the post office. = We have to go to the post office.

They can both be used to say that something is required by law:

You must file your taxes by April 15th. = You have to file your taxes by April 15th.

They can also both be used to say that something is very likely:

It must be 90 degrees in here. = It has to be 90 degrees in here.

Both have to and must can be used to add emphasis to a statement:

I must know, where did you get your shoes? = I have to know, where did you get your shoes?

Both words are also used to communicate a desire for something to happen, though have to is more commonly used this way in American English than must is:

You must come visit us. = You have to come visit us. [=We would like you to visit us.]

And finally, both have to and must can be used in questions or statements to convey anger or irritation, though with slightly different constructions:

Must you be so annoying? = Do you have to be so annoying?
Why must it rain on our wedding day? = Why does it have to rain on our wedding day?

In all cases above, must is more formal than have to.

There is, however, one more use of have to that is unique: if you have to do, finish, etc. (something) or have (something) to do, finish, etc., that means that the thing mentioned has to be done in order to complete a task:

I only have six more pages to read and then I'm done with the book.
I only have to read six more pages and then I'm done with the book.