We use different forms of the verb "must" to express different things:
For a necessity, an obligation or a certainity we use "must".
For a prohibition we use "mustn't".
When there is no obligation or no necessity, we use "don't have to" or "doesn't have to".

Necessity, obligation or certainity:
I be careful, when I cross the road.
You hurry up, if you want to be in time.

Prohibition:
He make noise in the middle of the night.
She smoke in the hospital.

No obligation or necessity:
We buy expensive clothes.
They go to the cinema.

Now use the correct form in the following sentences:
You learn the words, if you want to pass the test.
I get up early today, we leave at 5 o'clock a.m.
Paris be a most interesting city.
He smoke in the train, it isn't allowed.
I do this homework, it isn't compulsory.
She park here, it is a no-parking zone.
She dress nicely for the opera.
There are crocodiles in the water, you swim here.
The weather is very nice, she take an umbrella along.
He go to work, he's on a holiday.
We be late for school.
We get up early today, it's Sunday.