have to/must

[ponpoco256]

Hi I've another question. This time the question is about the semantic difference between have to and must in the sense of obligation. My grammar book (published in UK) says that must is used when the obligation comes from an internal need between the speaker and the listener while have to is used when the obligation comes from 'outside' - like a law, a rule or any order given by the third person.
(1) This is a terrible party. We really must go home.
(2) This is a lovely party, but we have to go home.
(3) My tooth aches. I must go to the dentist.
(4) I have to go to the dentist today because I made an appointment.
Is this rule also true in American English?

[Rusty]

1) This is a terrible party. We really must go home.
(2) This is a lovely party, but we have to go home.
(3) My tooth aches. I must go to the dentist.
(4) I have to go to the dentist today because I made an appointment.

Is this rule also true in American English?

(All these sentences sound natural, but I am not sure that the rule is applied strictly in American English. "I'll have to think about it" seems to defy the rule. Here are some websites to review:
http://www.englishpage.com/modals/haveto.html
http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/negmod.htm
http://www.collegeem.qc.ca/cemdept/anglais/musthavt.htm
http://www.learnenglish.org.uk/grammar/archive/mustvshaveto.html ).

[Pete]

Hi I've another question. This time the question is about the semantic difference between have to and must in the sense of obligation. My grammar book (published in UK) says that must is used when the obligation comes from an internal need between the speaker and the listener while have to is used when the obligation comes from 'outside' - like a law, a rule or any order given by the third person.
(1) This is a terrible party. We really must go home.
(2) This is a lovely party, but we have to go home.
(3) My tooth aches. I must go to the dentist.
(4) I have to go to the dentist today because I made an appointment.
Is this rule also true in American English?
I have a grammar book that says that (at least in speech) Americans tend to use "have to" much more often than "must" to show either external or internal obligation. In the case of external obligation, they almost invariably use "have to". In the case of internal obligation, "must" sometimes appears and doesn't sound unnatural, but "have to" is used most of the time.

For Americans, "must" is largely reserved for statements showing logical inference:
- He missed class today; he must be sick. (Because he always attends if he can.)
- You must be tired. (Because I know you have been working hard for a long time.)

[ponpoco256]

In the case of external obligation, Americans almost invariably use "have to". In the case of internal obligation, "must" sometimes appears and doesn't sound unnatural, but "have to" is used most of the time.
Rusty and Pete

You two are great teachers. Rusty, thank you for pasting the grammar sites. They are really helpful to learn the usage of modals. Pete, your explanation is very clear. I think I understand now the American way of using 'must' and 'have to'.

ponpoco

[teleostomi]

For Americans, "must" is largely reserved for statements showing logical inference:
- He missed class today; he must be sick. (Because he always attends if he can.)
- You must be tired. (Because I know you have been working hard for a long time.)

He should be sick.
He ought to be sick.
He must be sick.

Speaking of British English, do these three have the same meaning?