by chance 和 by accident 的差別在哪裡？如果不好說明，請提供一些例句。

They can be used to mean the same thing, but there are differences.
By chance usually means that something happens that nobody influenced. It just happens because it happens. I think the meaning is close to 偶然に or たまたま.
By accident means that someone tries to do something, but something else happens. The meaning is similar to しまう.
For example:
I found a 100 yen coin by chance. = 偶然に百円玉を見つけた。
I accidentally dropped the cup. = コップを落としてしまった。