disbelieve:
vt. 不信怀疑
vi. 不信怀疑
There is no reason to disbelievehim.
没理由不相信他。
doubt:
n. 怀疑疑问疑惑
v. 怀疑不信恐怕拿不准
There is no doubt that he is a hero of our country.
毫无疑问,他是我们国家的英雄。