there are always some traffic accidents happening easily when the road is ......为什么用happening?

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there are always some traffic accidents happening easily when the road is ......为什么用happening?

there are always some traffic accidents happening easily when the road is ......为什么用happening?
there are always some traffic accidents happening easily when the road is ......
为什么用happening?

there are always some traffic accidents happening easily when the road is ......为什么用happening?
there be 句型+动词ing形式;be 是be动词,happen 是动词,不能同时存在两个动词,所以happen 采用现在分词形式