You don't have to pay to use the park...it's free!
You can get up later today... it's Sunday!
They know they mustn't leave rubbish in the park.
You mustn't be late for the test!
There are signs everywhere saying that teenagers can't ride skateboards or bikes.
Everyone can enjoy the park.
They know they must keep their park clean.
We don't have to wear a uniform at school, thank God!
prohibition
it's not necessary
obligation
it is allowed/possible
it is not allowed
prohibition
it's not necessary
it is allowed