The reason the negatives are complicated is because they mean different things. "I have to" means you are required to do something. "I don't have to" is like a personal choice. "I must not" again means you are not allowed to do something. There is also "It is okay to" which is like getting permission. And there is "it is not okay to" which is like you are being told you must not do something. Sorry, I don't mean to complicate your simple question, but understanding the concepts of "must/have to/need to/don't need to/it's okay to/it's not okay to" are really important and use different forms.