Ask questions or just talk about different languages. Since BrainKing is an international game site supporting many languages, this board can be kind of useful.
Marfitalu: That was a nice link. That 'Many authorities consider that must indicates an internal decision of the speaker, while have to indicates the presumption of an outward authority' seems to be quite distinctive. How about Ought to?
I think that's another verb used to show obligation. And should?
I think the use of these four verbs must, have to, should, ought to and the one Walter mentioned, got to will be most obvious in examples. So do you have any in mind?