Must vs. Have To

Both "must" and "have to" express obligation, but they are used differently depending on who imposes the obligation.

Usage	Examples	Explanation
The speaker thinks it is necessary, or it is the rule.	"I must buy flowers for my mother."	(It's her birthday and I decide to do that.)
	"You must take more exercise," says the doctor. "It's necessary."	(The doctor thinks it is necessary.)
	"I must ask my secretary to book a flight for me."	(It is important for me not to forget.)
	"Dogs must be kept on a lead."	(Written on a sign in the park which must be obeyed.)
	"All pupils must wear a uniform."	(The headmaster says it is the rule.)
	"All passengers must wear seat belts."	(It's the law.)

Usage	Examples	Explanation
The obligation comes from an external source.	"I have to buy flowers for my mother-in-law."	(It is not my decision - my husband asked me to buy them.)
	"I have to take more exercise."	(The doctor told me it was necessary.)
	"I have to call the travel agency."	(My boss asked me to book a flight.)
	"I have to keep my dog on a lead."	(That's what the sign tells me to do.)
	"Do you have to wear a uniform in your school?"	(Is it compulsory?)
	"Do I have to fasten my seat belt?"	(Is it an obligation?)

Form	Meaning	Examples	Explanation
Mustn't	It is important not to do something; it is forbidden.	"You mustn't tell George."	(It is important not to tell George; don't tell George.)
		"Students mustn't talk during the exam."	(It is forbidden.)
		"You mustn't speak to strangers."	(Parents warn their children.)
		"You mustn't park here."	(Only residents can park here.)
Don't Have To	It is not necessary to do something; it is optional.	"You don't have to tell George."	(You can tell George if you like, but it isn't necessary. It's your decision.)
		"You don't have to wear a tie."	(But you can wear a tie if you like.)
		"You don't have to drive me home. I'll walk."	(It's not necessary. I live nearby.)
		"You don't have to whisper."	(It's not necessary. There's nobody else.)

"Must" can also be used to show that we are certain something is true. In this case, we are making a logical deduction based on the evidence we have.

Examples	Explanation
"Julie has had a long journey. She must be tired."	(Logical deduction based on the evidence of her long journey.)
"There's no heat in this room. You must be cold."	(Logical deduction based on the evidence of no heat.)
"Pablo hasn't eaten all day. He must be hungry."	(Logical deduction based on the evidence of not eating.)
"I didn't order these books. There must be a mistake."	(Logical deduction based on the evidence of not ordering the books.)

Tense	Usage	Examples
Present	'Must' only exists in the present tense.	"I must go to the meeting."
Past	'Have to' is used for past obligations.	"I had to go to see the dentist yesterday." (NOT: "I have must / I musted …")
Future	'Have to' is used for future obligations.	"I will have to get up early tomorrow." (NOT "I will must ….")

LEXONARY