when did it happen和 when has it happend?有什么区别

when did it happen和 when has it happend?有什么区别
when did it happen?
when has it happened?
有什么区别,具体要怎么翻译?
一个是一般过去时,一个是过去完成时,这两种时态之间又有怎样的相同点和不同点?