MUST and HAVE TO are both used to express an obligation, responsibility or necessity.

While Must can generally be replaced by Have to in the present tense, there is sometimes a slight difference in meaning or use.

Let’s look at our previous example using MUST:

I must write a letter to John.
We generally use must when the speaker decides that something is necessary, or needs to be done.

In this case I have decided that I need to write a letter to John. Nobody else has told me to write it. I think it is necessary.

The other example with HAVE TO.

I have to write a letter to John.
We use have to when somebody else other than the speaker has made the decision.

So here, I didn’t decide to write a letter. Somebody else has told me to write it, somebody else told me it was necessary to do.

Let’s look at some more examples:

I must book a hotel for my trip next week.
Here I am reminding myself that it is necessary to do. I have decided that it is necessary.

We have to wear a uniform at work.
Our boss makes us wear a uniform. It is an obligation that our boss has decided that is necessary. I, the speaker, am not making this obligation, someone else is.

Compare the following sentences:

The Teacher says: You must complete the essay by Friday
The Student says: We have to complete the essay by Friday.
The teacher has used MUST because he or has is giving the students an obligation.

Since the teacher has given us the obligation, we use have to. This is because somebody else, in this case the teacher, has told us what needs to be done.


When we mention someone else’s obligation, then we use the correct conjugation of Have to.

For example:

Mike can’t come because he has to work tomorrow.
It is Mike’s obligation to fulfill his work commitments.

Susan and Steve have to pay their rent every Friday.
They both have the obligation to pay their rent on that day.

In both examples you would not use MUST because we are talking about someone else’s obligations.
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