what has happen 和what was happen 和what happen有什么区别,它们后面都能加to吗?

题目

what has happen 和what was happen 和what happen有什么区别,它们后面都能加to吗?

答案

前两个句子都是错的!
1.what has happened 完成时态
2.what was happening 过去进行时态
第三个通常也应该说：what happened/happens 一般过去或者一般现在时态
都可以加to
三者只是时态不同,用法都一样.