Originally Posted by hela
Must to is used when you have the obligation coming from yourself, and have to when there is something external that obliges you to do something.
I must stop smoking once for all.
I have to abide by the rules of this forum.
One important difference to take into account is that "must not/ mustn't" does not mean that you don't have an obligation but that you are forbidden to do something (prohibition)
You must not smoke in here. It is forbidden.
I have not to do that homework today. I can do it tomorrow.
The question about "deduction"
Must: It is used mostly to express the conclusion that something is certain. This is used in British English.
Have to: Used in American English
You must be Anna's sister -you look just like her.
You have to be joking! (AmE) / You must be joking! (BrE)
In the past tense you can only use MUST
I hear you've been to Patagonia. That must have been interesting. (NOT)
"that has to have been interesting"
Hope this helps! Maeve