1、意思一样，用法不一样。
2、He accidently discovered the gold ore. He discovered the gold ore by accident. 一定放最后by chance 和 by accident 意思用法都一样。
3、happen to意思一样，用法为：He happened to discover the gold ore. Sb. happen(s/ed) to do sth. 求采纳～。
