Here, what's is the contraction of what has. The past participle of the main verb the meaning is. This is the present perfect, which is constructed using:
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What happened 和 what's happened两者的区别： 是问 what happened？只是想知道过去所发生的事情， 而如果用what's happened？则强调对现在的影响。，即想知道过去所发生的什么事导致了说话时.
The helping verb have 2.
'happend' is a spelling mistake of happened. 接下来让我们看下what happen和what happened的用法区别： 1.词性：what happen是用原形动词happen构成的现在时短语，而what happened是用动词happen的过去式happened构成的过去时短语. 表示“某地 (某时)发生了什么事”，常用“sth.+ happen +地点/时间”这一结构来表达，此时主语应是事情。例如： the story. 接下来让我们看下what happen和what happened的用法区别： 1.词性：what happen是用原形动词happen构成的现在时短语，而what happened是用动词happen的过去式happened构成的过.
