初三英语问题There is always something happening in the mall这句话中, h

There is always something happening in the mall
这句话中, happen为什么要变成happening?如果说正在进行时,为什么会有always?(always是一般现在时的标志）,难道说有There be always sth happening 这个词组? 不要跟我说补语状语定语啥的,我们老师没讲,请用别的方法讲一下!谢谢!
There is always something happening in the mall.中的happening in the mall是【现在分词短语】,在句中做定语,相当于一个定语从句.现在分词在此出现,确实超出了你们的范围.
There is always something happening in the mall.=There is always something that happens in the mall.