Hi! I've written a piece on English modal verbs in the 21st century, but it's too big to post here. The bottom line is that I've been teaching ESL in the Russian Federation for more than six years, and can't begin to understand why Russians are taught that there is really a difference between "must" and "have to." And it just floors me when I see people say "mustn't" and needn't." It's like, hello, are you Charles Dickens or what? I teach my students that there is NO difference between "have to" and "must." Besides being a native English speaker for more than 50 years,I know NO native English speakers who differentiate, and in the negative we say I don't have to or I don't need to. And the Oxford ESL publishing mafia continues to inculcate these crazy modal rules, but not all.