We use the verbs must and have to to express obligation. See underlined parts in the dialogue. – must and mustn’t express a strong obligation or strong recommendation
You must show your passport when you go through Passport Control.
You smoke far too much! You really must cut down. You mustn’tworry! Your husband will be alright.
– have to expresses a general obligation based on rules or what other people tell you to do.
Hospital staff have to wear uniforms.
The doctor told me I have to take two tablets a day with meals.
– We only use mustto refer to the present or immediate future.
Have to is used to make the past.
She had to have a tetanus injection when she cut her finger with the bread knife.
– must has no infinitive, -ing form or participles. So we make these form with have to.
You’ll have to stay in bed at least a week, I’m afraid.
Kate hates having to get up early.
I’ve had to cancel my holiday because of the cholera epidemic.