I would like to know the difference between may and might, must and have to ? How should I use them ? Thank you.
May is more probable than might, and may is used for permission more frequently than might.
Some that that with must the authority comes from the speaker/writer, but have to is an external authority:
You must do it (because I say so)
You have to do it (because it is a regulation)
This is a very simplified version of what are very complex verbs.