Must / Mustn't / Have To

A. MUST

• obligation - You must do your homework every day.
• almost certainty - Don't call her now. She must be busy.
• strong recommendation - You must try my chocolate cake recipe. You'll love it.

B. MUSTN'T

• prohibition - You mustn't litter the beach. / You mustn't eat in the classroom.

C. HAVE TO

• obligation or necessity - I have to be at the office by 8:30.

D. DON'T/ DOESN'T HAVE TO = DON'T / DOESN'T NEED TO

• absence of necessity - You don't have to buy more cheese. We've got plenty./ She doesn't have to get up early on Saturdays.

E. MUST vs HAVE TO

- MUST = personal obligation

I must go now. It's late.

We must finish this project today

- HAVE TO = obligation from outside imposed by other people

We have to wear a uniform at school.

She has to answer all the phone calls at work.

MUST is used only for the present to express what the speaker thinks is a personal obligation. In all other tenses we use HAVE TO

I must  go to the dentist today. My tooth is killing me.

I had to go to the dentist yesterday.

We'll have to work late next Friday.

Be careful!!!

MUST / MUSTN'T + BARE INFINITIVE

I must take the cat to the vet. NOT I must to take ... 

You mustn't smoke in this room. NOT You mustn't to smoke ....