You might want to double check that. :)Isaish spoke the words the Lord gave him to speak when Isaiah prophesized to the Hebrews. Since nothing in Scripture contradicts the prophecy that Isaiah spoke then it can be assured that he spoke the truth.
where do you see that recorded in the book of Jonah? Scripture says: Jonah 3:4 And Jonah began to enter into the city a day's journey, and he cried, and said, Yet forty days, and Nineveh shall be overthrown.. According to the standard you have set - Jonah is a false prophet.
In the case of Jonah, he was told if Nivevah repented of their sins then they would be spared. Since they did repent and were spared then the prophecy was true. God knowing all things, however, knew before hand that Nineveh would repent and thus added the stipulation when he gave the prophecy to Jonah.