To talk about obligations, we often use must and have to.
Let’s look at the differences between these and how we use them in conversation.
For affirmative statements, must and have to are very similar.
The difference depends on who feels the action is necessary.
Generally speaking, use must when the speaker feels something is necessary.
◆ I must book a hotel room. (I feel it is necessary to book a hotel room.)
◆ My doctor said, “You must quit smoking.” (My doctor feels it is necessary for me to quit smoking.)
On the other hand, use have to when somebody else thinks it is necessary. Compare the following examples:
◆ I have to book a hotel room. (My wife asked me to book a hotel room.)
◆ I have to quit smoking. (My doctor told me that it is necessary for me to quit smoking.)
For negative statements, must and have to have very different meanings.
Must not means that you should not do something.