impossible
Difficult, if not impossible
I've tried to open the box but it's impossible
cannot
I am sure he cannot do it
He can't sleep in a strange bed
no
'No left turn'
'No dumping'
must not
You mustn't touch exhibits
Nobody/no one leave the room!
Students are not allowed to talk during exams
He told me to lock you out; He told me not to open the door for you to come in
The computer's not supposed to make a noise like that
You are not to speak to strangers
not very
Not very good
Bread isn't as good as it used to be
unacceptable; inadmissible
dangerous; unsafe
The car is unsafe to drive
This toy/park is not safe for children