dubi

The preposition is the thing to fix in your head: you dubas pri something, never dubas ion. English hides the preposition, so the accusative feels natural and is wrong. Where a whole clause follows, use ĉu, because doubting is a question in disguise: mi dubas, ĉu li pravas. The noun dubo and the adverb sendube are both common, the second a mild irony machine, since sendube in argument usually announces that the speaker has no evidence. Dubinda means dubious, worth doubting, and is thoroughly rude about whatever it modifies.