Confusable words #9: know/understand/find out/discover
This sentence: “Yesterday I knew Eric Clapton was going to perform in Japan,” is not natural. It sounds like you already knew about it before yesterday. It is better to say: “Yesterday I found out that Eric Clapton was going to perform in Japan.” Remember, “know” is not a single point action, it is a continuous state, whereas “find out” is a single point action.
So, “found out” is the most common in this situation, but also “discovered”, “heard”, “read”, “saw in the newspaper” etc.
However, “discovered” has the nuance of something more unusual or surprising.
How to use “I knew”… an example would be, “He said he was busy but I knew that he had no appointments that day.” Another example: “When the new player was signed to the team, I was pleased because I knew he was very talented.” Using “had known” really emphasizes the fact that it was before. For example, “If I had known you were busy I wouldn’t have asked you to help me.”
“Understand” is used for something that may appear to be difficult , complex, or a special case. For example, “I understand that you are busy today, but would you mind helping me?”