Forms of verbs Modal verbs
To express what is possible or impossible you use:
I can ...
I could ...
I am able to ...
I'm in a position to ... or
I managed to get tickets.
To specify the probability of your choices or intentions you use:
I could ...
I might ...
I may ... or
I will go to a musical.
To express a recommendation or an obligation you use:
I should ...
I need to ... or
I must be on time.
And the negation is:
I needn't ... or
I don't have to wear something in particular.
The negation - "I mustn't bring a camera" means - no camera, it's forbidden!
I mustn't bring a camera! = NO camera!