Have to vs Must

[Andie]

I'm looking for clear examples to illustrate the difference between "have to" and "must". It is clear to me that you use "have to" when there's an obligation that is imposed from outside (laws-rules), whereas "must" expresses what the speaker thinks is necessary. But browsing through different books, I've found examples such as, "In England you must drive on the left." Examples like these ruin my explanation contrasting the external-internal point. I wonder if anyone can help me. Thanks!

[adilk]

Hi Andie

You ‘re right. “Have to” shows compulsion which imposed from outside. While “must” shows compulsion which comes from the speaker. But I found in a reference book that ‘must’ can be used to express deduction. We use it to say we are sure about something, because it is logically necessary. This is the only explanation I found for your example, which says ‘ In England you must drive on the left”. I hope my participation will add something to your good question. Thanks.

[GiddyGad]

Hi guys,

There's some connotation of possible unwillingness in "have to". The speaker either is aware of such or foresees its possibility.
"Must", on the contrary, doesn't take objections into account.

I hope this helps

GiddyGad

[Andie]

Thank you so much! What do you suggest I should do? Shall I stick to the "external-internal" contrast and give no further details? How do you go about this topic when you teach it?

[ihanna]

Hi guys,

There's some connotation of possible unwillingness in "have to". The speaker either is aware of such or foresees its possibility.
"Must", on the contrary, doesn't take objections into account.

I hope this helps

GiddyGad

Hello GiddyGad, I am also having some problems with must and have to. Can you please elaborate more? Like in this example:

“If your business keeps you so busy that you have no time for anything else, there must be something wrong either with you or with your business.” – William J. H. Boetcker

I am just confused in here it is like they can be exchanged freely. I mean what is the difference if I change the must in that quote to have to.

[GiddyGad]

In your example, the modal "must" has, as they call it, an epistemic meaning. Modal verbs have two meanings: epistemic and deontic. Say, "must" can express obligation (deontic) and probability, suggection, etc. (epistemic). Compare: "You must do it!" (deontic) and "You must be tired." (epistemic). "Have to" doesn't seem to have an epistemic meaning.

Good luck.

[ihanna]

Hello,
another question. What if you are telling someone "must" and you are just making an observation, like you must be tired, but then they think you are ordering them to do something? I totally understand the difference, but there must be some sentences where both meanings could be interpreted. I can't come up with any right now, but for example, maybe I tell someone "you must be John" and they think I am ordering them to be John? sorry, this is not a good example.

[GiddyGad]

In any case, the context (a facial expression, a gesture, a situation in general) should indicate what exactly you mean.
Here's my observation, which you can use in your practice: a Definite voice (Perfect, Continuous, Perfect Continuous), as opposed to Indefinite (or Simple, for that matter), after a modal verb will indicate the epistemic meaning of the modal. For example, "You must have done the job" will be interpreted as "I think that you have done the job"; "You will have heard the news" will mean "I take it you have already heard the news"; "He may be writing a test" will mean "Probably, he is writing a test at the moment", etc.
That is, at least in most cases it will be so.
Anyway, a language is not speaking the right words, but the ability to to verbally express yourself... and to understand what you are being told.