This is actually complex. Usually, we use present participle to indicate something that is currently happening: "He is going." "He is reading." However, 'know' is a special case. You either know something or you don't (or at least, you think you know." If you are in the process of knowing something, we would say "He is finding out that it is hard." or "He is learning that" 'Knowing' is used in connection with some other event: "He took the job, knowing that it was going to be a challenge." "He left, knowing that it would be a long time before he returned." In some cases, you can use 'knowing' as a noun. "The satisfaction is in (the) knowing you did a good job."