为什么有what happened却没有what happening,而只能是what is happening,两个动

来源：学生作业帮编辑：搜搜做题作业网作业帮分类：英语作业时间：2024/04/29 02:15:16

为什么有what happened却没有what happening,而只能是what is happening,两个动词能叠加一起?
最近钻牛角尖了,中学时代能理解的句子现在都掌控不了了.

what happened中的happened是过去式,可以直接做谓语,而happening是现在分词,是非谓语形式,不能充当谓语,但是可以和is之类的be动词一起构成进行时态来作谓语.
is happening中的is是助动词,是没有词义的.

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