"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