1.occur vi.发生,出现
解析：【同】happen,take place
【衍】occurrence n.发生
【辨析】happen强调偶然性
There happen to be no money with me.
It so happens that...
take place指有计划的使某事发生
Great changes have taken place in my hometown.
occur则包括了以上二者的意义,比较含糊.
2.occur是一般用词,可指某事情偶然发生或者按照计划进行,在表示具体失去的名词作主语的时候,可与happen换用,但比较正式,跟to短语时,表示“想到”,而不是“发生”,比如
I’m very sorry for all happened to you.
An idea occurs to me that going to Switzerland for traveling will be marvelous!