1. 请问have to do的否定形式是有don't have to do和needn't do两种吗？

【答】have to do的否定形式可用don't have to do或者 needn't do。

2. don't have to do和needn't do有区别吗？

【答】没有区别。例如：You needn't leave if you don't want to. [= you don't have to leave if you don't want to.]如果你不想离开，可用不必离开。You need not answer these questions. [= you don't have to answer these questions.]你不必回答这些问题。（此2例句摘自《韦氏高阶英汉双解词典》p.1387）