Difference between must and have to?

[Spearhead]

Hi Alan&Jamie (K), I remember reading a thread about the difference between must and have to. I know it must have been on the english-test.net forum but for some reason I can't find it any longer. I remember that you Alan and Jamie (K) talked about the different uses of must and have to in US and UK English. Could you possibly point me to that discussion again?
Thank you so much for your tireless support.
Olaf

[Jamie (K)]

I don't know where the discussion is, but I can tell you the basics. Alan can certainly complete whatever I've left out. Grammar books explain it, but they leave some things out.

Must and have to basically mean the same thing. However, must has to be in the auxiliary verb position:

You AUX[must] do this.

If you insert another auxiliary verb in that position, and must goes farther back in the sentence, then it is obligatory to use have to.

You AUX[will] have to do this.

Generally, in North American English, must is confined to strong, formal statements of obligation:

At the age of 18, all men must register for military service.

If you are not being so forceful, it is better to use have to when speaking to North Americans, or else you sound too dictatorial.

You may have noticed, however, that in British ESL textbooks they show people using must in very friendly situations, and this would not be normal in America. This mystified me until I listened to the recordings. I found out they are using a different pronunciation. Instead of pronouncing it with a full, strong vowel, as in [m^st], they said it with a very weak, quick schwa that is almost not there. It almost sounds like [mst]. This pronunciation does not sound dictatorial to Americans, but on the other hand we don't use it. Unless your pronunciation is very good, and you can say this [mst], I'd stay away from the word must with Americans unless you really are trying to be bossy.

I'm sure Alan will have some good insights on this.