This question already has an answer here:
- “I have to” vs. “I must” 2 answers
I am trying to teach the difference between the use have to or must. But my students do not understand anything. Could you help me?
Must and have to are synonymous, though some may prefer one or the other for certain purposes; this always happens with periphrastic modals, but these are merely individual preferences, not usage guides -- there is no consensus.
Must, as everyone knows, is a modal auxiliary verb.
So, when one needs to say things like these -- these are ungrammatical, but not unmeaningful after all -- one uses a periphrastic modal construction. Most modals have standard paraphrases that can be used in past and participial forms (be able to for alethic can, be allowed to for deontic may, ought to for should, etc).
The standard paraphrase for must turns out to be have to. Not only that, but there's a special pronunciation. This modal sense of the present tense phrases have to and has to
So what you can say instead of the starred sentences above is
But there's no meaning difference between them.