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To me "I must help her" sounds wrong. I feel that I would rarely say this.

On one website they say that 'must' is more for personal obligations (e.g. I must help my mother.) and that 'have to' is for external obligations (e.g. my boss says I have to finish the report.)

Is it that simple (and vague)? Or does anyone have a better explanation?

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up vote 3 down vote accepted

they say that 'must' is more for personal obligations.

That is not true; when somebody says "you must show your ID card," it is referring to an external obligation (e.g. by a law) not to an obligation you personally have because your ideas.

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Mmmm, that's a subtle distinction that I think the website is inventing or imagining. To my ear, "must" is simply more formal.

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I concur, on both counts. – Brian M. Scott Sep 15 '11 at 8:19
@Malvolio that's how I feel as well. I just don't want to step into a classroom and say "in my opinion" and have no proof that I am correct! – Istable Sep 15 '11 at 8:23
In matters of connotation, I don't know what else you can do -- short of assembling a panel of experts -- other than say "in my opinion, as a native speaker, ..." Actually, I just got an official english.SE t-shirt in the mail today, so I guess you could try, "According to a guy wearing a convincing t-shirt..." – Malvolio Sep 15 '11 at 8:30
@Malvolio: I want a t-shirt that says "According to a guy wearing a convincing t-shirt ..." – Peter Shor Sep 15 '11 at 13:12
Agreed; I would hardly ever use "must" in ordinary conversation, unless referring to an attic. – JeffSahol Sep 15 '11 at 13:19

For me "I have to" can transport a unwillingness to do the task.

Sorry I can't come. I must help my Mum.

This is more formal and the speaker is more or less happy to help.

Sorry I can't come. I have to help my Mum.

More informal and can express that the speaker isn't happy with helping.

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Agreed with this. – InfantPro'Aravind' Apr 2 '12 at 15:03

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