"Du må ikke kysse kua!"
Translation:You must not kiss the cow!
How do you distinguish between "You must not kiss the cow" and "You do not have to kiss the cow"?
You can use "Du er ikke nødt til å..." for the latter if you want to remove all doubt, but usually context and intonation does the job for you.
You'll have to ask him... if you can find him, that is. ;)