Re: must vs. have to
Here is the BrE system as I see it:
A. ‘Must’ suggests an internally felt obligation/necessity, ‘have to’ an externally imposed one:
Mother to son:
You must do your homework. (I say you must)
You have to do your homework. (Your teacher and/or your future success require this)
Swan and Murphy are British, which is why they say something similar.
Neither verb is inherently stronger or more formal than the other.
Three factors complicate the issue:
1. With first person 'I', people sometimes use 'have to' when there is in fact an inner necessity or self requirement. They do this to suggest that the requirement comes from an outside authority, in order to give the impression that they are reluctant to do what is required:
I have enjoyed talking to you, but I have to go.
2. Some people seem to switch in speech from one form to a stressed verison of the other in order to add emphasis. This is why you occasionally hear/see, "Must is stronger than have to" and "Have to is stronger than must". It's no wonder that learners are sometimes confused.
3. The negative forms of the two verbs convey very clearly different messages, as abaka pointed out. This difference in meanings is not present in the affirmative.
4. ‘Must’ has no past tense form. We frequently use ‘had to’ when one is required, blurring the difference in the present tense for some people.
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