As I know, the difference between "must" and "have to" is the former is more written while the latter is more spoken without any difference in meaning.
But there is an argument that "must" expresses the speaker's subjective willingness and command while "have to" expresses an objective binding obligation and "have to" is weaker in its meaning than "must"
But I doubt it, I think the two are interchangeable.
1. In my job I have to work from nine to five(more natural than "I must"?)
2. My husband's an interpreter. He often has to go to New York(more natural than "He must often?")
1. In my job I have to work from nine to five(more natural than "I must")
2. My husband's an interpreter. He often has to go to New York(more natural than "He must often")
These two are adopted from a grammar book, and they are not my ideas and I doubt them, I don't think they are the case.
Please let me know if the argument is true. Thanks!
There is no simple answer to the question, and you will find different books giving different advice. I have read, for example that must implies a stronger obligation than have to; I have also read that have to is stronger!
American colleagues tell me that must is rare in AmE, which suggests that Americans would not be aware of the difference I am about to mention.
I feel that in BrE, have to implies an externally imposed obligation, must an obligation imposed by the speaker.
1. Father to son: You have to do your homework.
2. Father to son: You must do your homework.
In #1, the father is thinking perhaps of the school rule;
In #2, the father is giving the order.
With I, the speaker may sometimes choose to use have to rather than must when there is a self-imposed (ie, speaker-imposed) obligation. This suggests that the speaker is forced by external obligations to do something over which s/he has no control.
Oh, it's 8 o'clock. I have to go. (The speaker is suggesting s/he has no choice. This implied reluctance to go is a social convention of politeness.)