Zero

Obligation in the present


must
we use must to express obligation
example
you must work hard
this sentence means that it is necessary that you work hard
the opposite of must in this sense is
don't have to =needn't
example

you don't have to drive fast .we have plenty of time

this sentence means that it is not necessary for you to drive fast becasue we have plenty of time


obligation in the past

had to

example

we had to walk since we missed teh bus.
This sentence means that it was necessary for us to walk becasue we missed the bus.

The opposite of had to is
didn't have to


she didnt have to go shooping .we have enough food at home .
This sentence means that it was not necessary for her to go shopping because we had enough food at home .

See u later with a new modal
good luck