"I think it's not easy.(X)" vs. "I don't think it's easy.(V)" (Grammar)
I think he will pass. I know that he will pass. I do not think he will pass. I know that he will not pass. I do not know whether he will pass. * I do not know (for certain) that he will pass./I do not know that he is a liar https://www.youtube.com/watch?v=r8HY5HAJmdQ
