I am Korean and have been studying English on my own for several years. Actually since I have no tutor so I have a hard time speaking English accurately and fluently though, that is not what I want to know for now.

My question is, What does it exactly mean by 'happen to do' ??

For example,

"I happen to know that." "Did you happen to let him know?" "How did you happen to hear that?"

I really want to know the difference between "I know that." and "I happen to know that."

I have taught there exists something different between them but usually failed to understand what it means.

I would really appreciate if you guys help me out and make me find that difference.

Thanks in advance. Have a good day then!! :)