MUST IS USED WHEN THERE ARE CONSEQUENCES IF YOU DON’T DO IT:

 

I must go to the dentist this afternoon.

You must never skip the lights. (=drive through red)

                                

MUST IS USED WHEN SOMETHING IS A LOGICAL CONCLUSION:

 

They have got a very big house, they must be very rich.

You must be John, Jonah’s brother. You look a lot like him

 

MUST = only present tense

IN OTHER TENSES YOU USE HAVE TO

 

PAST TENSE = HAD TO (NO AUXILIARY!!)

 

They had to visit gran yesterday.

DID they HAVE to visit gran yesterday?

They DIDN’T HAVE to visit gran.

 

 

She had to pay a lot of money.

DID she HAVE to pay a lot of money?

She DIDN’T HAVE to pay a lot of money.

 

FUTURE = WILL HAVE TO

 

They WILL HAVE to leave early tomorrow.

WILL they HAVE to leave early tomorrow?

They WON’T HAVE to leave early tomorrow