The meeting being held now is very important.

之前一直将这种 being done 当作“正在被……的”一种定语来背的，想具体请教这种结构是什么语法呢？或者说是哪种说话变化而来的。谢谢！

为回答如下：

(1) 以上结构为“现在分词一般式的被动语态”作后置定语修饰前面的名词：可以理解为“正在被做”

名词/代词+being done（现在分词被动式形成的分词短语结构）

=名词/代词 which is being done （定语从句结构）

(2) 现在分词被动语态作后置定语可以看成一个进行时态被动语态定语从句省略“主语+助动词”简化式结构

(1) The meeting being held now is very important

(分词被动式作后置定语)

(2)The meeting which is being held now is very important

正在被讨论的问题很重要。（定语从句进行时被动式作后置定语)

【1】现在分词的被动语态：一般式（being done）表示与谓语动词同时发生的被动的动作，完成式（having been done）表示发生在谓语动词之前的被动的动作。

The film being shown in the cinema is exciting. （作后置定语）

=The film which is being shown in the cinema is exciting.

正在这家上演的电影很棒。

The building being repaired is our library. （作后置定语）

正在维修的那座楼是我们的图书馆。

The problem being discussed is very important.（作后置定语）

正在被讨论的问题很重要。

I saw the man being taken away by the police. （作宾语补足语）

我看到那个人正被警察带走

【2】完成式（having been done）表示发生在谓语动词之前的被动的动作

Having been told many times, the naughty boy made the same mistake. 被告诉了好几遍，这个淘气的孩子又犯了同一个错误。

Having been asked to stay, I couldn’t very well leave.  既然要我留下，我就不好走了。

Having been bitten twice, the postman refused to deliver our letters unless we chained our dog up.  邮递员被狗咬了两次之后要我们把狗拴起来，不然就不给我们送信了。