When do we use 'must', when do we use 'have to'?

Must and have to both express obligation. However, they are used differently depending on who imposes the obligation.
MUST	HAVE TO
The speaker thinks it is necessary.	Someone else thinks it is necessary.
I must buy flowers for my mother. (It's her birthday and I decide to do that.)	I have to buy flowers for my mother-in-law. (It is not my decision - my husband asked me    to do it.)
"You must take more exercise" says the doctor. (The doctor thinks it is necessary.)	I have to take more exercise. (The doctor says it is necessary.)
I must ask my secretary to book a flight for me. (It is important for me not to forget.)	I have to call the travel agency. (My boss asked me to book a flight.)
"Dogs must be kept on a lead." (Written on a sign in the park = a rule which must be obeyed.)	I have to keep my dog on a lead. (That's what the sign tells me to do.)
N.B. In the negative form, the meaning changes.
You mustn't tell George = it is important not to tell George = don't tell George.	You don't have to tell George = you can tell George if you like, but it isn't necessary. It's your decision.